# Search ICLR 2019

Searching papers submitted to ICLR 2019 can be painful.
You might want to know which paper uses technique X, dataset D, or cites author ME.
Unfortunately, search is limited to titles, abstracts, and keywords, missing the actual *contents* of the paper.
This Frankensteinian search has returned from 2018 to help scour the papers of ICLR by ripping out their souls using `pdftotext`

.

Good luck! Warranty's not included :)

*Need random search inspiration..? Grab something from the list of all tags! ^_^*

How about:
deep learning theory,
variational inference,
resource efficient neural networks,
out-of-distribution inputs,
structural sparsity
..?

**Sanity Disclaimer:**
As you stare at the continuous stream of ICLR and arXiv papers,
don't lose confidence or feel overwhelmed.
This isn't a competition, it's a search for knowledge.
You and your work are valuable and help carve out the path for progress in our field :)

### "relu networks" has 53 results

##### Bayesian Convolutional Neural Networks with Many Channels are Gaussian Processes

**tl;dr** Finite-width SGD trained CNNs vs. infinitely wide fully Bayesian CNNs. Who wins?
(Show abstract)

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There is a previously identified equivalence between wide fully connected neural networks (FCNs) and Gaussian processes (GPs). This equivalence enables, for instance, test set predictions that would have resulted from a fully Bayesian, infinitely wide trained FCN to be computed without ever instantiating an FCN, but by instead evaluating the corresponding GP. In this work, we derive an analogous equivalence for multi-layer convolutional neural networks (CNNs) both with and without pooling layers. Surprisingly, in the absence of pooling layers, the corresponding GP is identical for CNNs with and without weight sharing. This means that translation equivariance in SGD-trained finite CNNs has no corresponding property in the Bayesian treatment of the infinite-width limit -- a qualitative difference between the two regimes that is not present in the FCN case. We confirm experimentally that in some scenarios, while the performance of trained finite CNNs becomes similar to that of the corresponding GP with increasing channel count, with careful tuning SGD-trained CNNs can significantly outperform their corresponding GPs. Finally, we introduce a Monte Carlo method to estimate the GP corresponding to a NN architecture, even in cases where the analytic form has too many terms to be computationally feasible.
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##### Fixing Variational Bayes: Deterministic Variational Inference for Bayesian Neural Networks

**tl;dr** A method for eliminating gradient variance and automatically tuning priors for effective training of bayesian neural networks
(Show abstract)

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Bayesian neural networks (BNNs) hold great promise as a flexible and principled solution to deal with uncertainty when learning from finite data. Among approaches to realize probabilistic inference in deep neural networks, variational Bayes (VB) is theoretically grounded, generally applicable, and computationally efficient. With wide recognition of potential advantages, why is it that variational Bayes has seen very limited practical use for BNNs in real applications? We argue that variational inference in neural networks is fragile: successful implementations require careful initialization and tuning of prior variances, as well as controlling the variance of Monte Carlo gradient estimates. We fix VB and turn it into a robust inference tool for Bayesian neural networks. We achieve this with two innovations: first, we introduce a novel deterministic method to approximate moments in neural networks, eliminating gradient variance; second, we introduce a hierarchical prior for parameters and a novel Empirical Bayes procedure for automatically selecting prior variances. Combining these two innovations, the resulting method is highly efficient and robust. On the application of heteroscedastic regression we demonstrate strong predictive performance over alternative approaches.
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##### A Walk with SGD: How SGD Explores Regions of Deep Network Loss?

No tl;dr =[ (Show abstract)

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The non-convex nature of the loss landscape of deep neural networks (DNN) lends them the intuition that over the course of training, stochastic optimization algorithms explore different regions of the loss surface by entering and escaping many local minima due to the noise induced by mini-batches. But is this really the case? This question couples the geometry of the DNN loss landscape with how stochastic optimization algorithms like SGD interact with it during training. Answering this question may help us qualitatively understand the dynamics of deep neural network optimization. We show evidence through qualitative and quantitative experiments that mini-batch SGD rarely crosses barriers during DNN optimization. As we show, the mini-batch induced noise helps SGD explore different regions of the loss surface using a seemingly different mechanism. To complement this finding, we also investigate the qualitative reason behind the slowing down of this exploration when using larger batch-sizes. We show this happens because gradients from larger batch-sizes align more with the top eigenvectors of the Hessian, which makes SGD oscillate in the proximity of the parameter initialization, thus preventing exploration.
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##### Understanding GANs via Generalization Analysis for Disconnected Support

**tl;dr** We investigate the generalization performance of GANs and show how GANs outperform others with a specific property of data.
(Show abstract)

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This paper provides theoretical analysis of generative adversarial networks (GANs) to explain its advantages over other standard methods of learning probability measures. GANs learn a probability through observations, using the objective function with a generator and a discriminator. While many empirical results indicate that GANs can generate realistic samples, the reason for such successful performance remains unelucidated. This paper focuses the situation where the target probability measure satisfies the disconnected support property, which means a separate support of a probability, and relates it with the advantage of GANs. It is theoretically shown that, unlike other popular models, GANs do not suffer from the decrease of generalization performance caused by the disconnected support property. We rigorously quantify the generalization performance of GANs of a given architecture, and compare it with the performance of the other models. Based on the theory, we also provide a guideline for selecting deep network architecture for GANs. We demonstrate some numerical examples which support our results.
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##### Training for Faster Adversarial Robustness Verification via Inducing ReLU Stability

**tl;dr** We develop methods to train deep neural models that are both robust to adversarial perturbations and whose robustness is significantly easier to verify.
(Show abstract)

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We explore the concept of co-design in the context of neural network verification. Specifically, we aim to train deep neural networks that not only are robust to adversarial perturbations but also whose robustness can be verified more easily. To this end, we identify two properties of network models - weight sparsity and so-called ReLU stability - that turn out to significantly impact the complexity of the corresponding verification task. We demonstrate that improving weight sparsity alone already enables us to turn computationally intractable verification problems into tractable ones. Then, improving ReLU stability leads to an additional 4-13x speedup in verification times. An important feature of our methodology is its "universality," in the sense that it can be used with a broad range of training procedures and verification approaches.
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##### ROBUST ESTIMATION VIA GENERATIVE ADVERSARIAL NETWORKS

**tl;dr** GANs are shown to provide us a new effective robust mean estimate against agnostic contaminations with both statistical optimality and practical tractability.
(Show abstract)

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Robust estimation under Huber's $\epsilon$-contamination model has become an important topic in statistics and theoretical computer science. Rate-optimal procedures such as Tukey's median and other estimators based on statistical depth functions are impractical because of their computational intractability. In this paper, we establish an intriguing connection between f-GANs and various depth functions through the lens of f-Learning. Similar to the derivation of f-GAN, we show that these depth functions that lead to rate-optimal robust estimators can all be viewed as variational lower bounds of the total variation distance in the framework of f-Learning. This connection opens the door of computing robust estimators using tools developed for training GANs. In particular, we show that a JS-GAN that uses a neural network discriminator with at least one hidden layer is able to achieve the minimax rate of robust mean estimation under Huber's $\epsilon$-contamination model. Interestingly, the hidden layers of the neural net structure in the discriminator class are shown to be necessary for robust estimation.
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##### Neural Networks with Structural Resistance to Adversarial Attacks

**tl;dr** We introduce a type of neural network that is structurally resistant to adversarial attacks, even when trained on unaugmented training sets. The resistance is due to the stability of network units wrt input perturbations.
(Show abstract)

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In adversarial attacks to machine-learning classifiers, small perturbations are added to input that is correctly classified. The perturbations yield adversarial examples, which are virtually indistinguishable from the unperturbed input, and yet are misclassified. In standard neural networks used for deep learning, attackers can craft adversarial examples from most input to cause a misclassification of their choice.
We introduce a new type of network units, called RBFI units, whose non-linear structure makes them inherently resistant to adversarial attacks. On permutation-invariant MNIST, in absence of adversarial attacks, networks using RBFI units match the performance of networks using sigmoid units, and are slightly below the accuracy of networks with ReLU units. When subjected to adversarial attacks, networks with RBFI units retain accuracies above 93% for projected gradient descent (PGD) attacks that degrade the accuracy of networks with ReLU or sigmoid units to below 70%.Considering a variety of attack mechanisms, RBFI networks trained on regular input either exceed or closely match the accuracy of sigmoid and ReLU network trained with the help of adversarial examples.
The non-linear structure of RBFI units makes them difficult to train using standard gradient descent. We show that RBFI networks of RBFI units can be efficiently trained to high accuracies using pseudogradients, computed using functions especially crafted to facilitate learning instead of their true derivatives.
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##### Geometry of Deep Convolutional Networks

**tl;dr** Analysis of deep convolutional networks in terms of associated arrangement of hyperplanes
(Show abstract)

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We give a formal procedure for computing preimages of convolutional
network outputs using the dual basis defined from the set of
hyperplanes associated with the layers of the network. We point out
the special symmetry associated with arrangements of hyperplanes of
convolutional networks that take the form of regular
multidimensional polyhedral cones. We discuss the efficiency of of
large number of layers of nested cones that result from incremental
small size convolutions in order to give a good compromise between
efficient contraction of data to low dimensions and shaping of
preimage manifolds. We demonstrate how a specific network flattens a
non linear input manifold to an affine output manifold and discuss
it's relevance to understanding classification properties of deep
networks.
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##### The Nonlinearity Coefficient - Predicting Generalization in Deep Neural Networks

**tl;dr** We introduce the NLC, a metric that is cheap to compute in the networks randomly initialized state and is highly predictive of generalization, at least in fully-connected networks.
(Show abstract)

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For a long time, designing neural architectures that exhibit high performance was considered a dark art that required expert hand-tuning. One of the few well-known guidelines for architecture design is the avoidance of exploding or vanishing gradients. However, even this guideline has remained relatively vague and circumstantial, because there exists no well-defined, gradient-based metric that can be computed {\it before} training begins and can robustly predict the performance of the network {\it after} training is complete.
We introduce what is, to the best of our knowledge, the first such metric: the nonlinearity coefficient (NLC). Via an extensive empirical study, we show that the NLC, computed in the network's randomly initialized state, is a powerful predictor of test error and that attaining a right-sized NLC is essential for attaining an optimal test error, at least in fully-connected feedforward networks. The NLC is also conceptually simple, cheap to compute, and is robust to a range of confounders and architectural design choices that comparable metrics are not necessarily robust to. Hence, we argue the NLC is an important tool for architecture search and design, as it can robustly predict poor training outcomes before training even begins.
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##### The role of over-parametrization in generalization of neural networks

**tl;dr** We suggest a generalization bound that could potentially explain the improvement in generalization with over-parametrization.
(Show abstract)

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Despite existing work on ensuring generalization of neural networks in terms of scale sensitive complexity measures, such as norms, margin and sharpness, these complexity measures do not offer an explanation of why neural networks generalize better with over-parametrization. In this work we suggest a novel complexity measure based on unit-wise capacities resulting in a tighter generalization bound for two layer ReLU networks. Our capacity bound correlates with the behavior of test error with increasing network sizes, and could potentially explain the improvement in generalization with over-parametrization. We further present a matching lower bound for the Rademacher complexity that improves over previous capacity lower bounds for neural networks.
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##### Adaptivity of deep ReLU network for learning in Besov and mixed smooth Besov spaces: optimal rate and curse of dimensionality

No tl;dr =[ (Show abstract)

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Deep learning has shown high performances in various types of tasks from visual recognition to natural language processing,
which indicates superior flexibility and adaptivity of deep learning.
To understand this phenomenon theoretically, we develop a new approximation and estimation error analysis of
deep learning with the ReLU activation for functions in a Besov space and its variant with mixed smoothness.
The Besov space is a considerably general function space including the Holder space and Sobolev space, and especially can capture spatial inhomogeneity of smoothness. Through the analysis in the Besov space, it is shown that deep learning can achieve the minimax optimal rate and outperform any non-adaptive (linear) estimator such as kernel ridge regression,
which shows that deep learning has higher adaptivity to the spatial inhomogeneity of the target function than other estimators such as linear ones. In addition to this, it is shown that deep learning can avoid the curse of dimensionality if the target function is in a mixed smooth Besov space. We also show that the dependency of the convergence rate on the dimensionality is tight due to its minimax optimality. These results support high adaptivity of deep learning and its superior ability as a feature extractor.
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##### The Unreasonable Effectiveness of (Zero) Initialization in Deep Residual Learning

**tl;dr** All you need to train deep residual networks is a good initialization; normalization layers are not necessary.
(Show abstract)

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Normalization layers are a staple in state-of-the-art deep neural network architectures. They are widely believed to stabilize training, enable higher learning rate, accelerate convergence and improve generalization, though the reason for their effectiveness is still an active research topic. In this work, we challenge the commonly-held beliefs by showing that none of the perceived benefits is unique to normalization. Specifically, we propose ZeroInit, an initialization motivated by solving the exploding and vanishing gradient problem at the beginning of training by initializing as a zero function. We find training residual networks with ZeroInit to be as stable as training with normalization - even for networks with 10,000 layers. Furthermore, with proper regularization, ZeroInit without normalization matches or exceeds the performance of state-of-the-art residual networks in image classification and machine translation.
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##### On the Selection of Initialization and Activation Function for Deep Neural Networks

**tl;dr** How to effectively choose Initialization and Activation function for deep neural networks
(Show abstract)

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The weight initialization and the activation function of deep neural networks have a crucial impact on the performance of the training procedure. An inappropriate selection can lead to the loss of information of the input during forward propagation and the exponential vanishing/exploding of gradients during back-propagation. Understanding the theoretical properties of untrained random networks is key to identifying which deep networks may be trained successfully as recently demonstrated by Schoenholz et al. (2017) who showed that for deep feedforward neural networks only a specific choice of hyperparameters known as the `edge of chaos' can lead to good performance.
We complete this analysis by providing quantitative results showing that, for a class of ReLU-like activation functions, the information propagates indeed deeper for an initialization at the edge of chaos. By further extending this analysis, we identify a class of activation functions that improve the information propagation over ReLU-like functions. This class includes the Swish activation, $\phi_{swish}(x) = x \cdot \text{sigmoid}(x)$, used in Hendrycks & Gimpel (2016),
Elfwing et al. (2017) and Ramachandran et al. (2017). This provides a theoretical grounding for the excellent empirical performance of $\phi_{swish}$ observed in these contributions. We complement those previous results by illustrating the benefit of using a random initialization on the edge of chaos in this context.
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##### On the Geometry of Adversarial Examples

**tl;dr** We present a geometric framework for proving robustness guarantees and highlight the importance of codimension in adversarial examples.
(Show abstract)

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Adversarial examples are a pervasive phenomenon of machine learning models where seemingly imperceptible perturbations to the input lead to misclassifications for otherwise statistically accurate models. We propose a geometric framework, drawing on tools from the manifold reconstruction literature, to analyze the high-dimensional geometry of adversarial examples. In particular, we highlight the importance of codimension: for low-dimensional data manifolds embedded in high-dimensional space there are many directions off the manifold in which to construct adversarial examples. Adversarial examples are a natural consequence of learning a decision boundary that classifies the low-dimensional data manifold well, but classifies points near the manifold incorrectly. Using our geometric framework we prove (1) a tradeoff between robustness under different norms, (2) that adversarial training in balls around the data is sample inefficient, and (3) sufficient sampling conditions under which nearest neighbor classifiers and ball-based adversarial training are robust.
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##### On the Margin Theory of Feedforward Neural Networks

**tl;dr** We show that training feedforward relu networks with a weak regularizer results in a maximum margin and analyze the implications of this result.
(Show abstract)

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Past works have shown that, somewhat surprisingly, over-parametrization can help generalization in neural networks. Towards explaining this phenomenon, we adopt a margin-based perspective. We establish: 1) for multi-layer feedforward relu networks, the global minimizer of a weakly-regularized cross-entropy loss has the maximum normalized margin among all networks, 2) as a result, increasing the over-parametrization improves the normalized margin and generalization error bounds for two-layer networks. In particular, an infinite-size neural network enjoys the best generalization guarantees. The typical infinite feature methods are kernel methods; we compare the neural net margin with that of kernel methods and construct natural instances where kernel methods have much weaker generalization guarantees. We validate this gap between the two approaches empirically. Finally, this infinite-neuron viewpoint is also fruitful for analyzing optimization. We show that a perturbed gradient flow on infinite-size networks finds a global optimizer in polynomial time.
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##### Detecting Memorization in ReLU Networks

**tl;dr** We use the non-negative rank of ReLU activation matrices as a complexity measure and show it (negatively) correlates with good generalization.
(Show abstract)

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We propose a new notion of 'non-linearity' of a network layer with respect to an input batch that is based on its proximity to a linear system, which is reflected in the non-negative rank of the activation matrix.
We measure this non-linearity by applying non-negative factorization to the activation matrix.
Considering batches of similar samples, we find that high non-linearity in deep layers is indicative of memorization. Furthermore, by applying our approach layer-by-layer, we find that the mechanism for memorization consists of distinct phases. We perform experiments on fully-connected and convolutional neural networks trained on several image and audio datasets. Our results demonstrate that as an indicator for memorization, our technique can be used to perform early stopping.
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##### GenEval: A Benchmark Suite for Evaluating Generative Models

**tl;dr** We introduce battery of synthetic distributions and metrics for measuring the success of generative models
(Show abstract)

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Generative models are important for several practical applications, from low level image processing tasks, to model-based planning in robotics. More generally,
the study of generative models is motivated by the long-standing endeavor to model uncertainty and to discover structure by leveraging unlabeled data.
Unfortunately, the lack of an ultimate task of interest has hindered progress in the field, as there is no established way to
compare models and, often times, evaluation is based on mere visual inspection of samples drawn from such models.
In this work, we aim at addressing this problem by introducing a new benchmark evaluation suite, dubbed \textit{GenEval}.
GenEval hosts a large array of distributions capturing many important
properties of real datasets, yet in a controlled setting, such as lower intrinsic dimensionality, multi-modality, compositionality,
independence and causal structure. Any model can be easily plugged for evaluation, provided it can generate samples.
Our extensive evaluation suggests that different models have different strenghts, and that GenEval is a great tool to gain insights about how models and metrics work.
We offer GenEval to the community~\footnote{Available at: \it{coming soon}.} and believe that this benchmark will facilitate comparison and development of
new generative models.
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##### Gradient descent aligns the layers of deep linear networks

No tl;dr =[ (Show abstract)

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This paper establishes risk convergence and asymptotic weight matrix alignment --- a form of implicit regularization --- of gradient flow and gradient descent when applied to deep linear networks on linearly separable data. In more detail, for gradient flow applied to strictly decreasing loss functions (with similar results for gradient descent with particular decreasing step sizes):
(1) the risk converges to 0;
(ii) the normalized i-th weight matrix asymptotically equals its
rank-1 approximation u_iv_i^T;
(iii) these rank-1 matrices are aligned across layers, meaning |v_{i+1}^T u_i| -> 1.
In the case of the logistic loss (binary cross entropy), more can be said: the linear function induced by the network --- the product of its weight matrices --- converges to the same direction as the maximum margin solution. This last property was identified in prior work, but only under assumptions on gradient descent which here are implied by the alignment phenomenon.
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##### On the loss landscape of a class of deep neural networks with no bad local valleys

No tl;dr =[ (Show abstract)

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We identify a class of over-parameterized deep neural networks with standard activation functions and cross-entropy loss which provably have no bad local valley, in the sense that from any point in parameter space there exists a continuous path on which the cross-entropy loss is non-increasing and gets arbitrarily close to zero. This implies that these networks have no sub-optimal strict local minima.
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##### Robustness Certification with Refinement

**tl;dr** We refine the over-approximation results from incomplete verifiers using MILP solvers to prove more robustness properties than state-of-the-art.
(Show abstract)

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We present a novel approach for verification of neural networks which combines scalable over-approximation methods with precise (mixed integer) linear programming. This results in significantly better precision than state of the art verifiers on feed forward neural networks with piecewise linear activation functions.
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##### How Training Data Affect the Accuracy and Robustness of Image Classification Models

No tl;dr =[ (Show abstract)

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Recent work has demonstrated the lack of robustness of well-trained deep neural networks (DNNs) to adversarial examples. For example,
visually indistinguishable perturbations, when mixed with an original
image, can easily lead deep learning models to misclassifications. In
light of a recent study on the mutual influence between robustness and
accuracy over 18 different ImageNet models, this paper investigates
how training data affect the accuracy and robustness of deep neural
networks. We conduct extensive experiments on four different datasets,
including CIFAR-10, MNIST, STL-10, and Tiny ImageNet, with several
representative neural networks. Our results reveal previously unknown
phenomena that exist between the size of training data and
characteristics of the resulting models. In particular, we find that
model accuracy improves monotonically with increased training
data. Similarly, model robustness also improves, but starts to
deteriorate when training data continue to increase. The occurrence of
turning points depends on the deep neural network as well as the
dataset on which it is trained.
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##### How Training Data Affect the Accuracy and Robustness of Neural Networks for Image Classification

No tl;dr =[ (Show abstract)

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Recent work has demonstrated the lack of robustness of well-trained deep neural networks (DNNs) to adversarial examples. For example, visually indistinguishable perturbations, when mixed with an original image, can easily lead deep learning models to misclassifications. In light of a recent study on the mutual influence between robustness and accuracy over 18 different ImageNet models, this paper investigates how training data affect the accuracy and robustness of deep neural
networks. We conduct extensive experiments on four different datasets, including CIFAR-10, MNIST, STL-10, and Tiny ImageNet, with several representative neural networks. Our results reveal previously unknown phenomena that exist between the size of training data and characteristics of the resulting models. In particular, besides confirming that the model accuracy improves as the amount of training data increases, we also observe that the model robustness improves initially, but there exists a turning point after which robustness starts to decrease. How and when such turning points occur vary for different neural networks and different datasets.
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##### Subgradient Descent Learns Orthogonal Dictionaries

**tl;dr** Efficient dictionary learning by L1 minimization via a novel analysis of the non-convex non-smooth geometry.
(Show abstract)

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This paper concerns dictionary learning, viz., sparse coding, a fundamental representation learning problem. We show that a subgradient descent algorithm, with random initialization, can recover orthogonal dictionaries on a natural nonsmooth, nonconvex L1 minimization formulation of the problem, under mild statistical assumption on the data. This is in contrast to previous provable methods that require either expensive computation or delicate initialization schemes. Our analysis develops several tools for characterizing landscapes of nonsmooth functions, which might be of independent interest for provable training of deep networks with nonsmooth activations (e.g., ReLU), among other applications. Preliminary experiments corroborate our analysis and show that our algorithm works well empirically in recovering orthogonal dictionaries.
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##### Approximation and non-parametric estimation of ResNet-type convolutional neural networks via block-sparse fully-connected neural networks

**tl;dr** It is shown that ResNet-type CNNs are a universal approximator and its expression ability is not worse than fully connected neural networks (FNNs) with a \textit{block-sparse} structure even if the size of each layer in the CNN is fixed.
(Show abstract)

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We develop new approximation and statistical learning theories of convolutional neural networks (CNNs) via the ResNet-type structure where the channel size, width, and filter size are fixed. It is shown that a ResNet-type CNN is a universal approximator and its expression ability is no worse than fully connected neural networks (FNNs) with a \textit{block-sparse} structure even if the size of each layer in the CNN is fixed. Our result is general in the sense that we can automatically translate any approximation rate achieved by block-sparse FNNs into that by CNNs. Thanks to the general theory, it is shown that learning on CNNs satisfies optimality in approximation and estimation of several important function classes.
As applications, we consider two types of function classes to be estimated: the Barron class and the H\"older class. We prove the regularized empirical risk minimization (ERM) estimator can achieve the same rate as FNNs even the channel size, filter size, and width of CNNs are constant with respect to the sample size. This is minimax optimal (up to logarithmic factors) for the H\"older class. Our proof is based on sophisticated evaluations of the covering number of CNNs and the non-trivial parameter rescaling technique to control the Lipschitz constant of CNNs to be constructed.
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##### Estimating Information Flow in DNNs

**tl;dr** Deterministic deep neural networks do not discard information, but they do cluster their inputs.
(Show abstract)

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We study the evolution of internal representations during deep neural network (DNN) training, aiming to demystify the compression aspect of the information bottleneck theory. The theory suggests that DNN training comprises a rapid fitting phase followed by a slower compression phase, in which the mutual information I(X;T) between the input X and internal representations T decreases. Several papers observe compression of estimated mutual information on different DNN models, but the true I(X;T) over these networks is provably either constant (discrete X) or infinite (continuous X). This work explains the discrepancy between theory and experiments, and clarifies what was actually measured by these past works. To this end, we introduce an auxiliary (noisy) DNN framework for which I(X;T) is a meaningful quantity that depends on the network's parameters. This noisy framework is shown to be a good proxy for the original (deterministic) DNN both in terms of performance and the learned representations. We then develop a rigorous estimator for I(X;T) in noisy DNNs and observe compression in various models. By relating I(X;T) in the noisy DNN to an information-theoretic communication problem, we show that compression is driven by the progressive clustering of hidden representations of inputs from the same class. Several methods to directly monitor clustering of hidden representations, both in noisy and deterministic DNNs, are used to show that meaningful clusters form in the T space. Finally, we return to the estimator of I(X;T) employed in past works, and demonstrate that while it fails to capture the true (vacuous) mutual information, it does serve as a measure for clustering. This clarifies the past observations of compression and isolates the geometric clustering of hidden representations as the true phenomenon of interest.
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##### Evaluating Robustness of Neural Networks with Mixed Integer Programming

**tl;dr** We efficiently verify the robustness of deep neural models with over 100,000 ReLUs, certifying more samples than the state-of-the-art and finding more adversarial examples than a strong first-order attack.
(Show abstract)

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Neural networks trained only to optimize for training accuracy can often be fooled by adversarial examples --- slightly perturbed inputs misclassified with high confidence. Verification of networks enables us to gauge their vulnerability to such adversarial examples. We formulate verification of piecewise-linear neural networks as a mixed integer program. On a representative task of finding minimum adversarial distortions, our verifier is two to three orders of magnitude quicker than the state-of-the-art. We achieve this computational speedup via tight formulations for non-linearities, as well as a novel presolve algorithm that makes full use of all information available. The computational speedup allows us to verify properties on convolutional and residual networks with over 100,000 ReLUs --- several orders of magnitude more than networks previously verified by any complete verifier. In particular, we determine for the first time the exact adversarial accuracy of an MNIST classifier to perturbations with bounded l-∞ norm ε=0.1: for this classifier, we find an adversarial example for 4.38% of samples, and a certificate of robustness to norm-bounded perturbations for the remainder. Across all robust training procedures and network architectures considered, and for both the MNIST and CIFAR-10 datasets, we are able to certify more samples than the state-of-the-art and find more adversarial examples than a strong first-order attack.
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##### How Training Data Affect the Accuracy and Robustness of Neural Networks for Image Classification

No tl;dr =[ (Show abstract)

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Recent work has demonstrated the lack of robustness of well-trained deep neural networks (DNNs) to adversarial examples. For example, visually indistinguishable perturbations, when mixed with an original image, can easily lead deep learning models to misclassifications. In light of a recent study on the mutual influence between robustness and accuracy over 18 different ImageNet models, this paper investigates how training data affect the accuracy and robustness of deep neural
networks. We conduct extensive experiments on four different datasets, including CIFAR-10, MNIST, STL-10, and Tiny ImageNet, with several representative neural networks. Our results reveal previously unknown phenomena that exist between the size of training data and characteristics of the resulting models. In particular, besides confirming that the model accuracy improves as the amount of training data increases, we also observe that the model robustness improves initially, but there exists a turning point after which robustness starts to deteriorate. How and when such turning points occur vary for different neural networks and different datasets.
*

##### Verification of Non-Linear Specifications for Neural Networks

No tl;dr =[ (Show abstract)

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Prior work on neural network verification has focused on specifications that are linear functions of the output of the network, e.g., invariance of the classifier output under adversarial perturbations of the input. In this paper, we extend verification algorithms to be able to certify richer properties of neural networks. To do this we introduce the class of convex-relaxable specifications, which constitute nonlinear specifications that can be verified using a convex relaxation. We show that a number of important properties of interest can be modeled within this class, including conservation of energy in a learned dynamics model of a physical system; semantic consistency of a classifier's output labels under adversarial perturbations and bounding errors in a system that predicts the summation of handwritten digits. Our experimental evaluation shows that our method is able to effectively verify these specifications. Moreover, our evaluation exposes the failure modes in models which cannot be verified to satisfy these specifications. Thus, emphasizing the importance of training models not just to fit training data but also to be consistent with specifications.
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##### Deterministic PAC-Bayesian generalization bounds for deep networks via generalizing noise-resilience

**tl;dr** We provide a PAC-Bayes based generalization guarantee for deep networks by generalizing noise-resilience of the network on the training data to the test data.
(Show abstract)

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The ability of overparameterized deep networks to generalize well has been linked to the fact that stochastic gradient descent (SGD) finds solutions that lie in flat, wide minima in the training loss -- minima where the output of the network is resilient to small random noise added to its parameters.
So far this observation has been used to provide generalization guarantees only for neural networks whose parameters are either \textit{stochastic} or \textit{compressed}. In this work, we present a general PAC-Bayesian framework that leverages this observation to provide a bound on the original network learned -- a network that is deterministic and uncompressed. What enables us to do this is a key novelty in our approach: our framework allows us to show that if on training data, the interactions between the weight matrices satisfy certain conditions that imply a wide training loss minimum, these conditions themselves {\em generalize} to the interactions between the matrices on test data, thereby implying a wide test loss minimum. We then apply our general framework in a setup where we assume that the pre-activation values of the network are not too small (although we assume this only on the training data). In this setup, we provide a generalization guarantee for the original (deterministic, uncompressed) network, that does not scale with product of the spectral norms of the weight matrices -- a guarantee that would not have been possible with prior approaches.
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##### When Will Gradient Methods Converge to Max-margin Classifier under ReLU Models?

**tl;dr** We study the implicit bias of gradient methods in solving a binary classification problem with nonlinear ReLU models.
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We study the implicit bias of gradient descent methods in solving a binary classification problem over a linearly separable dataset. The classifier is described by a nonlinear ReLU model and the objective function adopts the exponential loss function. We first characterize the landscape of the loss function and show that there can exist spurious asymptotic local minima besides asymptotic global minima. We then show that gradient descent (GD) can converge to either a global or a local max-margin direction, or may diverge from the desired max-margin direction in a general context. For stochastic gradient descent (SGD), we show that it converges in expectation to either the global or the local max-margin direction if SGD converges. We further explore the implicit bias of these algorithms in learning a multi-neuron network under certain stationary conditions, and show that the learned classifier maximizes the margins of each sample pattern partition under the ReLU activation.
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##### The Limitations of Adversarial Training and the Blind-Spot Attack

**tl;dr** We show that the effectiveness of adversarial training procedure on test set has a strong correlation with the distance between the test point and the manifold of training data.
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The adversarial training procedure proposed by Madry et. al. is one of the most effective methods to defend against adversarial examples on deep neuron networks (DNNs). Despite being very effective on MNIST, adversarial training on larger datasets like CIFAR and ImageNet achieves much worse results. In our paper, we shed some lights on the practicality and hardness of adversarial training by first showing that the effectiveness of adversarial training procedure on test set has a strong correlation with the distance between the test point and the manifold of training data. The test examples that are relatively far away from the distribution of training dataset are more likely to be vulnerable to adversarial examples. Consequentially, adversarial training based defense is susceptible to a new class of attacks (“blind-spot attack”) where the input image resides in a “blind-spot” in the empirical distribution of training data but is still on the ground-truth data manifold. For MNIST, we found that these blind-spots can be easily found by simply scaling and shifting image pixel values. Most importantly, for large datasets with high dimensional and complex data manifold (CIFAR, ImageNet, etc), the existence of blind-spots in adversarial training makes the defense on any valid test examples almost impossible due to the curse of dimensionality.
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##### Efficiently testing local optimality and escaping saddles for ReLU networks

**tl;dr** A theoretical algorithm for testing local optimality and extracting descent directions at nondifferentiable points of empirical risks of one-hidden-layer ReLU networks.
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We provide a theoretical algorithm for checking local optimality and escaping saddles at nondifferentiable points of empirical risks of two-layer ReLU networks. Our algorithm receives any parameter value and returns: local minimum, second-order stationary point, or a strict descent direction. The presence of M data points on the nondifferentiability of the ReLU divides the parameter space into at most 2^M regions, which makes analysis difficult. By exploiting polyhedral geometry, we reduce the total computation down to one convex quadratic program (QP) for each hidden node, O(M) (in)equality tests, and one (or a few) nonconvex QP. For the last QP, we show that our specific problem can be solved efficiently, in spite of nonconvexity. In the benign case, we solve one equality constrained QP, and we prove that projected gradient descent solves it exponentially fast. In the bad case, we have to solve a few more inequality constrained QPs, but we prove that the time complexity is exponential only in the number of inequality constraints. Our experiments show that either benign case or bad case with very few inequality constraints occurs, implying that our algorithm is efficient in most cases.
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##### Statistical Characterization of Deep Neural Networks and their Sensitivity

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Despite their ubiquity, it remains an active area of research to fully understand deep neural networks (DNNs) and the reasons of their empirical success. We contribute to this effort by introducing a principled approach to statistically characterize DNNs and their sensitivity. By distinguishing between randomness from input data and from model parameters, we study how central and non-central moments of network activation and sensitivity evolve during propagation. Thereby, we provide novel statistical insights on the hypothesis space of input-output mappings encoded by different architectures. Our approach applies both to fully-connected and convolutional networks and incorporates most ingredients of modern DNNs: rectified linear unit (ReLU) activation, batch normalization, skip connections.
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##### ProxQuant: Quantized Neural Networks via Proximal Operators

**tl;dr** A principled framework for model quantization using the proximal gradient method.
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To make deep neural networks feasible in resource-constrained environments (such as mobile devices), it is beneficial to quantize models by using low-precision weights. One common technique for quantizing neural networks is the straight-through gradient method, which enables back-propagation through the quantization mapping. Despite its empirical success, little is understood about why the straight-through gradient method works.
Building upon a novel observation that the straight-through gradient method is in fact identical to the well-known Nesterov’s dual-averaging algorithm on a quantization constrained optimization problem, we propose a more principled alternative approach, called ProxQuant , that formulates quantized network training as a regularized learning problem instead and optimizes it via the prox-gradient method. ProxQuant does back-propagation on the underlying full-precision vector and applies an efficient prox-operator in between stochastic gradient steps to encourage quantizedness. For quantizing ResNets and LSTMs, ProxQuant outperforms state-of-the-art results on binary quantization and is on par with state-of-the-art on multi-bit quantization. For binary quantization, our analysis shows both theoretically and experimentally that ProxQuant is more stable than the straight-through gradient method (i.e. BinaryConnect), challenging the indispensability of the straight-through gradient method and providing a powerful alternative.
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##### Combinatorial Attacks on Binarized Neural Networks

**tl;dr** Gradient-based attacks on binarized neural networks are not effective due to the non-differentiability of such networks; Our IPROP algorithm solves this problem using integer optimization
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Binarized Neural Networks (BNNs) have recently attracted significant interest due to their computational efficiency. Concurrently, it has been shown that neural networks may be overly sensitive to "attacks" -- tiny adversarial changes in the input -- which may be detrimental to their use in safety-critical domains. Designing attack algorithms that effectively fool trained models is a key step towards learning robust neural networks.
The discrete, non-differentiable nature of BNNs, which distinguishes them from their full-precision counterparts, poses a challenge to gradient-based attacks. In this work, we study the problem of attacking a BNN through the lens of combinatorial and integer optimization. We propose a Mixed Integer Linear Programming (MILP) formulation of the problem. While exact and flexible, the MILP quickly becomes intractable as the network and perturbation space grow. To address this issue, we propose IProp, a decomposition-based algorithm that solves a sequence of much smaller MILP problems. Experimentally, we evaluate both proposed methods against the standard gradient-based attack (FGSM) on MNIST and Fashion-MNIST, and show that IProp performs favorably compared to FGSM, while scaling beyond the limits of the MILP.
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##### Decoupling Gating from Linearity

**tl;dr** We propose Gated Linear Unit networks — a model that performs similarly to ReLU networks on real data while being much easier to analyze theoretically.
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The gap between the empirical success of deep learning and the lack of strong theoretical guarantees calls for studying simpler models. By observing that a ReLU neuron is a product of a linear function with a gate (the latter determines whether the neuron is active or not), where both share a jointly trained weight vector, we propose to decouple the two. We introduce GaLU networks — networks in which each neuron is a product of a Linear Unit, defined by a weight vector which is being trained, with a Gate, defined by a different weight vector which is not being trained. Generally speaking, given a base model and a simpler version of it, the two parameters that determine the quality of the simpler version are whether its practical performance is close enough to the base model and whether it is easier to analyze it theoretically. We show that GaLU networks perform similarly to ReLU networks on standard datasets and we initiate a study of their theoretical properties, demonstrating that they are indeed easier to analyze. We believe that further research of GaLU networks may be fruitful for the development of a theory of deep learning.
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##### On the Universal Approximability and Complexity Bounds of Quantized ReLU Neural Networks

**tl;dr** This paper proves the universal approximability of quantized ReLU neural networks and puts forward the complexity bound given arbitrary error.
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Compression is a key step to deploy large neural networks on resource-constrained platforms. As a popular compression technique, quantization constrains the number of distinct weight values and thus reducing the number of bits required to represent and store each weight. In this paper, we study the representation power of quantized neural networks. First, we prove the universal approximability of quantized ReLU networks on a wide class of functions. Then we provide upper bounds on the number of weights and the memory size for a given approximation error bound and the bit-width of weights for function-independent and function-dependent structures. Our results reveal that, to attain an approximation error bound of $\epsilon$, the number of weights needed by a quantized network is no more than $\mathcal{O}\left(\log^5(1/\epsilon)\right)$ times that of an unquantized network. This overhead is of much lower order than the lower bound of the number of weights needed for the error bound, supporting the empirical success of various quantization techniques. To the best of our knowledge, this is the first in-depth study on the complexity bounds of quantized neural networks.
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##### The Expressive Power of Deep Neural Networks with Circulant Matrices

**tl;dr** We provid a theoretical study of the properties of Deep circulant-diagonal ReLU Networks and demonstrated that they are bounded width universal approximators.
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Recent results from linear algebra stating that any matrix can be decomposed into products of diagonal and circulant matrices has lead to the design of compact deep neural network architectures that perform well in practice. In this paper, we bridge the gap between these good empirical results
and the theoretical approximation capabilities of Deep diagonal-circulant ReLU networks. More precisely, we first demonstrate that a Deep diagonal-circulant ReLU networks of
bounded width and small depth can approximate a deep ReLU network in which the dense matrices are
of low rank. Based on this result, we provide new bounds on the expressive power and universal approximativeness of this type of networks. We support our experimental results with thorough experiments on a large, real world video classification problem.
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##### Adaptive Estimators Show Information Compression in Deep Neural Networks

**tl;dr** We developed robust mutual information estimates for DNNs and used them to observe compression in networks with non-saturating activation functions
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To improve how neural networks function it is crucial to understand their learning process. The information bottleneck theory of deep learning proposes that neural networks achieve good generalization by compressing their representations to disregard information that is not relevant for the task. However, empirical evidence for this theory is conflicting, as compression was only observed when the networks used a saturating activation functions. In contrast, networks with non-saturating activation functions achieved comparable levels of task performance but did not show compression. In this paper we developed a more robust mutual information estimation technique, that adapts to hidden activity of neural networks and produces more sensitive measurements of activations from all functions, especially unbounded functions. Using these adaptive estimation techniques, we explored compression in networks with a range of different activation functions. With two improved methods of estimation, firstly, we show that saturation of the activation function is not required for compression, and the amount of compression varies between different activation functions. We also found that there is a large amount of variation in compression between different network initializations. Secondary, we see that L2 regularization leads to significantly increased compression, while preventing overfitting. Finally, we show that only compression of the last layer is positively correlated with generalization.
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##### A Priori Estimates of the Generalization Error for Two-layer Neural Networks

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New estimates for the generalization error are established for a nonlinear regression problem using a two-layer neural network model. These new estimates are a priori in nature in the sense that the bounds depend only on some norms of the underlying functions to be fitted, not the parameters in the model. In contrast, most existing results for neural networks are a posteriori in nature in the sense that the bounds depend on some norms of the model parameters. The error rates are comparable to that of the Monte Carlo method in terms of the size of the dataset. Moreover, these bounds are equally effective in the over-parametrized regime when the network size is much larger than the size of the dataset.
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##### Towards Robust, Locally Linear Deep Networks

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Deep networks realize complex mappings that are often understood by their locally linear behavior around or at points of interest. For example, we use the derivative of the mapping with respect to its inputs for sensitivity analysis, or to explain (obtain coordinate relevance for) a prediction. One key challenge is that such derivates are themselves inherently unstable. In this paper, we propose a new learning problem to encourage deep networks to have stable derivatives over larger regions. While the problem is challenging in general, we focus on networks with piecewise linear activation functions. Our algorithm consists of an inference step that identifies a region around a point where linear approximation is provably stable, and an optimization step to expand such regions. We propose a novel relaxation to scale the algorithm to realistic models. We demonstrate algorithm and the resulting solutions with residual and recurrent networks on image and sequence datasets.
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##### How Training Data Affect the Accuracy and Robustness of Neural Networks for Image Classification

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Recent work has demonstrated the lack of robustness of well-trained deep neural networks (DNNs) to adversarial examples. For example, visually indistinguishable perturbations, when mixed with an original image, can easily lead deep learning models to misclassifications. In light of a recent study on the mutual influence between robustness and accuracy over 18 different ImageNet models, this paper investigates how training data affect the accuracy and robustness of deep neural
networks. We conduct extensive experiments on four different datasets, including CIFAR-10, MNIST, STL-10, and Tiny ImageNet, with several representative neural networks. Our results reveal previously unknown phenomena that exist between the size of training data and characteristics of the resulting models. In particular, besides confirming that the model accuracy improves as the amount of training data increases, we also observe that the model robustness improves initially, but there exists a turning point after which robustness starts to deteriorate. How and when such turning points occur vary for different neural networks and different datasets.
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##### Invariance and Inverse Stability under ReLU

**tl;dr** We analyze the invertibility of deep neural networks by studying preimages of ReLU-layers and the stability of the inverse.
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We flip the usual approach to study invariance and robustness of neural networks by considering the non-uniqueness and instability of the inverse mapping. We provide theoretical and numerical results on the inverse of ReLU-layers. First, we derive a necessary and sufficient condition on the existence of invariance that provides a geometric interpretation. Next, we move to robustness via analyzing local effects on the inverse. To conclude, we show how this reverse point of view not only provides insights into key effects, but also enables to view adversarial examples from different perspectives.
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##### G-SGD: Optimizing ReLU Neural Networks in its Positively Scale-Invariant Space

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It is well known that neural networks with rectified linear units (ReLU) activation functions are positively scale-invariant. Conventional algorithms like stochastic gradient descent optimize the neural networks in the vector space of weights, which is, however, not positively scale-invariant. This mismatch may lead to problems during the optimization process. Then, a natural question is: \emph{can we construct a new vector space that is positively scale-invariant and sufficient to represent ReLU neural networks so as to better facilitate the optimization process }? In this paper, we provide our positive answer to this question. First, we conduct a formal study on the positive scaling operators which forms a transformation group, denoted as $\mathcal{G}$. We prove that the value of a path (i.e. the product of the weights along the path) in the neural network is invariant to positive scaling and the value vector of all the paths is sufficient to represent the neural networks under mild conditions. Second, we show that one can identify some basis paths out of all the paths and prove that the linear span of their value vectors (denoted as $\mathcal{G}$-space) is an invariant space with lower dimension under the positive scaling group. Finally, we design stochastic gradient descent algorithm in $\mathcal{G}$-space (abbreviated as $\mathcal{G}$-SGD) to optimize the value vector of the basis paths of neural networks with little extra cost by leveraging back-propagation. Our experiments show that $\mathcal{G}$-SGD significantly outperforms the conventional SGD algorithm in optimizing ReLU networks on benchmark datasets.
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##### Collapse of deep and narrow neural nets

**tl;dr** Deep and narrow neural networks will converge to erroneous mean or median states of the target function depending on the loss with high probability.
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Recent theoretical work has demonstrated that deep neural networks have superior performance over shallow networks, but their training is more difficult, e.g., they suffer from the vanishing gradient problem. This problem can be typically resolved by the rectified linear unit (ReLU) activation. However, here we show that even for such activation, deep and narrow neural networks will converge to erroneous mean or median states of the target function depending on the loss with high probability. We demonstrate this collapse of deep and narrow neural networks both numerically and theoretically, and provide estimates of the probability of collapse. We also construct a diagram of a safe region of designing neural networks that avoid the collapse to erroneous states. Finally, we examine different ways of initialization and normalization that may avoid the collapse problem.
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##### The Universal Approximation Power of Finite-Width Deep ReLU Networks

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We show that finite-width deep ReLU neural networks yield rate-distortion optimal approximation (Bölcskei et al., 2018) of a wide class of functions, including polynomials, windowed sinusoidal functions, one-dimensional oscillatory textures, and the Weierstrass function, a fractal function which is continuous but nowhere differentiable. Together with the recently established universal approximation result for affine function systems (Bölcskei et al., 2018), this demonstrates that deep neural networks approximate vastly different signal structures generated by the affine group, the Weyl-Heisenberg group, or through warping, and even certain fractals, all with approximation error decaying exponentially in the number of neurons. We also prove that in the approximation of sufficiently smooth functions finite-width deep networks require strictly fewer neurons than finite-depth wide networks.
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##### On the Spectral Bias of Neural Networks

**tl;dr** We investigate ReLU networks in the Fourier domain and demonstrate peculiar behaviour.
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Prior work has theoretically established neural networks as a class of highly expressive functions. Their ability to memorize even random input-output mapping with 100% accuracy can be seen as a practical implication of this aspect. In this work we present properties of neural networks that complement this aspect of expressivity. By using tools from Fourier analysis to study neural networks, We show that deep ReLU networks are biased towards low frequency functions, meaning that, they cannot have local fluctuations without affecting their global behavior. Intuitively, this property is in line with the observation that over-parameterized networks find simple patterns that generalize across data samples. We also investigate how the shape of the data manifold affects this spectral bias by showing strong evidence that different manifold shapes induce significantly different learning curves for deep ReLU networks and present a theoretical understanding of this behavior. Finally, we study the robustness of parameters to develop the intuition that parameters of a network must work together to express high frequency functions.
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##### The Comparative Power of ReLU Networks and Polynomial Kernels in the Presence of Sparse Latent Structure

**tl;dr** Beyond-worst-case analysis of the representational power of ReLU nets & polynomial kernels -- in particular in the presence of sparse latent structure.
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There has been a large amount of interest, both in the past and particularly recently, into the relative advantage of different families of universal function approximators, for instance neural networks, polynomials, rational functions, etc. However, current research has focused almost exclusively on understanding this problem in a worst case setting: e.g. characterizing the best L1 or L_{infty} approximation in a box (or sometimes, even under an adversarially constructed data distribution.) In this setting many classical tools from approximation theory can be effectively used.
However, in typical applications we expect data to be high dimensional, but structured -- so, it would only be important to approximate the desired function well on the relevant part of its domain, e.g. a small manifold on which real input data actually lies. Moreover, even within this domain the desired quality of approximation may not be uniform; for instance in classification problems, the approximation needs to be more accurate near the decision boundary. These issues, to the best of our knowledge, have remain unexplored until now.
With this in mind, we analyze the performance of neural networks and polynomial kernels in a natural regression setting where the data enjoys sparse latent structure, and the labels depend in a simple way on the latent variables. We give an almost-tight theoretical analysis of the performance of both neural networks and polynomials for this problem, as well as verify our theory with simulations. Our results both involve new (complex-analytic) techniques, which may be of independent interest, and show substantial qualitative differences with what is known in the worst-case setting.
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##### Stochastic Gradient Descent Learns State Equations with Nonlinear Activations

**tl;dr** We study the state equation of a recurrent neural network. We show that SGD can efficiently learn the unknown dynamics from few input/output observations under proper assumptions.
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We study discrete time dynamical systems governed by the state equation $h_{t+1}=ϕ(Ah_t+Bu_t)$. Here A,B are weight matrices, ϕ is an activation function, and $u_t$ is the input data. This relation is the backbone of recurrent neural networks (e.g. LSTMs) which have broad applications in sequential learning tasks. We utilize stochastic gradient descent to learn the weight matrices from a finite input/state trajectory $(u_t,h_t)_{t=0}^N$. We prove that SGD estimate linearly converges to the ground truth weights while using near-optimal sample size. Our results apply to increasing activations whose derivatives are bounded away from zero. The analysis is based on i) an SGD convergence result with nonlinear activations and ii) careful statistical characterization of the state vector. Numerical experiments verify the fast convergence of SGD on ReLU and leaky ReLU in consistence with our theory.
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##### Small nonlinearities in activation functions create bad local minima in neural networks

**tl;dr** We constructively prove that even the slightest nonlinear activation functions introduce spurious local minima, for general datasets and activation functions.
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We investigate the loss surface of neural networks. We prove that even for one-hidden-layer networks with "slightest" nonlinearity, the empirical risks have spurious local minima in most cases. Our results thus indicate that in general "no spurious local minima" is a property limited to deep linear networks, and insights obtained from linear networks are not robust. Specifically, for ReLU(-like) networks we constructively prove that for almost all (in contrast to previous results) practical datasets there exist infinitely many local minima. We also present a counterexample for more general activations (sigmoid, tanh, arctan, ReLU, etc.), for which there exists a bad local minimum. Our results make the least restrictive assumptions relative to existing results on local optimality in neural networks. We complete our discussion by presenting a comprehensive characterization of global optimality for deep linear networks, which unifies other results on this topic.
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##### Deep, Skinny Neural Networks are not Universal Approximators

**tl;dr** This paper proves that skinny neural networks cannot approximate certain functions, no matter how deep they are.
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In order to choose a neural network architecture that will be effective for a particular modeling problem, one must understand the limitations imposed by each of the potential options. These limitations are typically described in terms of information theoretic bounds, or by comparing the relative complexity needed to approximate example functions between different architectures. In this paper, we examine the topological constraints that the architecture of a neural network imposes on the level sets of all the functions that it is able to approximate. This approach is novel for both the nature of the limitations and the fact that they are independent of network depth for a broad family of activation functions.
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##### Deep learning generalizes because the parameter-function map is biased towards simple functions

**tl;dr** The parameter-function map of deep networks is hugely biased; this can explain why they generalize. We use PAC-Bayes and Gaussian processes to obtain nonvacuous bounds.
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Deep neural networks generalize remarkably well without explicit regularization even in the strongly over-parametrized regime. This success suggests that some form of implicit regularization must be at work. In this paper we argue that a strong intrinsic bias in the parameter-function map helps explain the success of deep neural networks. We provide evidence that the parameter-function map results in a heavily biased prior over functions, if we assume that the training algorithm samples parameters close to uniformly within the zero-error region. The PAC-Bayes theorem then guarantees good expected generalization for target functions producing high-likelihood training sets.
We exploit connections between deep neural networks and Gaussian processes to estimate the marginal likelihood, finding remarkably good agreement between Gaussian processes and neural networks for small input sets. Using approximate marginal likelihood calculations we produce nontrivial generalization PAC-Bayes error bounds which correlate well with the true error on realistic datasets such as MNIST and CIFAR and for architectures including convolutional and fully connected networks.
As predicted by recent arguments based on algorithmic information theory, we find that the prior probability drops exponentially with linear increases in several measures of descriptional complexity of the target function. As target functions in many real problems are expected to be highly structured, this simplicity bias offers an insight into why deep networks generalize well on real world problems, but badly on randomized data.
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##### Universal Lipschitz Functions

**tl;dr** We identify pathologies in existing activation functions when learning neural networks with Lipschitz constraints and use these insights to design neural networks which are universal Lipschitz function approximators.
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Training neural networks with a Lipschitz constraint provides improved generalization, robustness, and interpretability. However, existing techniques either fail to guarantee a Lipschitz constraint or are unable to universally approximate Lipschitz functions. Often, a small Lipschitz constant is enforced by considering constraints on the network weights, but little attention is payed to the choice of activation function. We identify Jacobian norm of network layers as a scarce resource in representing Lipschitz functions and show that common activation functions are unable to effectively utilize this. We show that with common activation functions networks are unable to learn even the simplest Lipschitz functions, such as the absolute value function. With this insight, we introduce a novel activation function, the GroupSort activation, which partitions the hidden layer and sorts the units within each partition. Empirically, we identify pathologies of common activation functions and confirm that these theoretical observations are relevant in practice.
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