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Livni, Roi


Graph-based Discriminators: Sample Complexity and Expressiveness

Neural Information Processing Systems

A basic question in learning theory is to identify if two distributions are identical when we have access only to examples sampled from the distributions. This basic task is considered, for example, in the context of Generative Adversarial Networks (GANs), where a discriminator is trained to distinguish between a real-life distribution and a synthetic distribution. Classically, we use a hypothesis class $H$ and claim that the two distributions are distinct if for some $h\in H$ the expected value on the two distributions is (significantly) different. Our starting point is the following fundamental problem: "is having the hypothesis dependent on more than a single random example beneficial". To address this challenge we define $k$-ary based discriminators, which have a family of Boolean $k$-ary functions $\G$.


Can Implicit Bias Explain Generalization? Stochastic Convex Optimization as a Case Study

arXiv.org Machine Learning

The notion of implicit bias, or implicit regularization, has been suggested as a means to explain the surprising generalization ability of modern-days overparameterized learning algorithms. This notion refers to the tendency of the optimization algorithm towards a certain structured solution that often generalizes well. Recently, several papers have studied implicit regularization and were able to identify this phenomenon in various scenarios. We revisit this paradigm in arguably the simplest non-trivial setup, and study the implicit bias of Stochastic Gradient Descent (SGD) in the context of Stochastic Convex Optimization. As a first step, we provide a simple construction that rules out the existence of a \emph{distribution-independent} implicit regularizer that governs the generalization ability of SGD. We then demonstrate a learning problem that rules out a very general class of \emph{distribution-dependent} implicit regularizers from explaining generalization, which includes strongly convex regularizers as well as non-degenerate norm-based regularizations. Certain aspects of our constructions point out to significant difficulties in providing a comprehensive explanation of an algorithm's generalization performance by solely arguing about its implicit regularization properties.


An Equivalence Between Private Classification and Online Prediction

arXiv.org Machine Learning

We prove that every concept class with finite Littlestone dimension can be learned by an (approximate) differentially-private algorithm. This answers an open question of Alon et al. (STOC 2019) who proved the converse statement (this question was also asked by Neel et al.~(FOCS 2019)). Together these two results yield an equivalence between online learnability and private PAC learnability. We introduce a new notion of algorithmic stability called "global stability" which is essential to our proof and may be of independent interest. We also discuss an application of our results to boosting the privacy and accuracy parameters of differentially-private learners.


Prediction with Corrupted Expert Advice

arXiv.org Machine Learning

Prediction with expert advice is perhaps the single most fundamental problem in online learning and sequential decision making. In this problem, the goal of a learner is to aggregate decisions from multiple experts and achieve performance that approaches that of the best individual expert in hindsight. The standard performance criterion is the regret: the difference between the loss of the learner and that of the best single expert. The experts problem is often considered in the so-called adversarial setting, where the losses of the individual experts may be virtually arbitrary and even be chosen by an adversary so as to maximize the learner's regret. The canonical algorithm in this setup is the Multiplicative Weights algorithm (Littlestone and Warmuth, 1989; Freund and Schapire, 1995), that guarantees an optimal regret of Θ( T log N) in any problem with N experts and T decision rounds. A long line of research in online learning has focused on obtaining better regret guarantees, often referred to as "fast rates," on benign problem instances in which the loss generation process behaves more favourably than in a fully adversarial setup. A prototypical example of such an instance is the stochastic setting of the experts problem, where the losses of the experts are drawn i.i.d.


Affine-Invariant Online Optimization and the Low-rank Experts Problem

Neural Information Processing Systems

We present a new affine-invariant optimization algorithm called Online Lazy Newton. The regret of Online Lazy Newton is independent of conditioning: the algorithm's performance depends on the best possible preconditioning of the problem in retrospect and on its \emph{intrinsic} dimensionality. As an application, we show how Online Lazy Newton can be used to achieve an optimal regret of order $\sqrt{rT}$ for the low-rank experts problem, improving by a $\sqrt{r}$ factor over the previously best known bound and resolving an open problem posed by Hazan et al (2016). Papers published at the Neural Information Processing Systems Conference.


Multi-Armed Bandits with Metric Movement Costs

Neural Information Processing Systems

We consider the non-stochastic Multi-Armed Bandit problem in a setting where there is a fixed and known metric on the action space that determines a cost for switching between any pair of actions. The loss of the online learner has two components: the first is the usual loss of the selected actions, and the second is an additional loss due to switching between actions. Our main contribution gives a tight characterization of the expected minimax regret in this setting, in terms of a complexity measure $\mathcal{C}$ of the underlying metric which depends on its covering numbers. In finite metric spaces with $k$ actions, we give an efficient algorithm that achieves regret of the form $\widetilde(\max\set{\mathcal{C} {1/3}T {2/3},\sqrt{kT}})$, and show that this is the best possible. Our regret bound generalizes previous known regret bounds for some special cases: (i) the unit-switching cost regret $\widetilde{\Theta}(\max\set{k {1/3}T {2/3},\sqrt{kT}})$ where $\mathcal{C} \Theta(k)$, and (ii) the interval metric with regret $\widetilde{\Theta}(\max\set{T {2/3},\sqrt{kT}})$ where $\mathcal{C} \Theta(1)$.


On the Computational Efficiency of Training Neural Networks

Neural Information Processing Systems

It is well-known that neural networks are computationally hard to train. On the other hand, in practice, modern day neural networks are trained efficiently using SGD and a variety of tricks that include different activation functions (e.g. ReLU), over-specification (i.e., train networks which are larger than needed), and regularization. In this paper we revisit the computational complexity of training neural networks from a modern perspective. We provide both positive and negative results, some of them yield new provably efficient and practical algorithms for training neural networks.


Graph-based Discriminators: Sample Complexity and Expressiveness

arXiv.org Machine Learning

A basic question in learning theory is to identify if two distributions are identical when we have access only to examples sampled from the distributions. This basic task is considered, for example, in the context of Generative Adversarial Networks (GANs), where a discriminator is trained to distinguish between a real-life distribution and a synthetic distribution. % Classically, we use a hypothesis class $H$ and claim that the two distributions are distinct if for some $h\in H$ the expected value on the two distributions is (significantly) different. Our starting point is the following fundamental problem: "is having the hypothesis dependent on more than a single random example beneficial". To address this challenge we define $k$-ary based discriminators, which have a family of Boolean $k$-ary functions $\mathcal{G}$. Each function $g\in \mathcal{G}$ naturally defines a hyper-graph, indicating whether a given hyper-edge exists. A function $g\in \mathcal{G}$ distinguishes between two distributions, if the expected value of $g$, on a $k$-tuple of i.i.d examples, on the two distributions is (significantly) different. We study the expressiveness of families of $k$-ary functions, compared to the classical hypothesis class $H$, which is $k=1$. We show a separation in expressiveness of $k+1$-ary versus $k$-ary functions. This demonstrate the great benefit of having $k\geq 2$ as distinguishers. For $k\geq 2$ we introduce a notion similar to the VC-dimension, and show that it controls the sample complexity. We proceed and provide upper and lower bounds as a function of our extended notion of VC-dimension.


On the Expressive Power of Kernel Methods and the Efficiency of Kernel Learning by Association Schemes

arXiv.org Machine Learning

We study the expressive power of kernel methods and the algorithmic feasibility of multiple kernel learning for a special rich class of kernels. Specifically, we define \emph{Euclidean kernels}, a diverse class that includes most, if not all, families of kernels studied in literature such as polynomial kernels and radial basis functions. We then describe the geometric and spectral structure of this family of kernels over the hypercube (and to some extent for any compact domain). Our structural results allow us to prove meaningful limitations on the expressive power of the class as well as derive several efficient algorithms for learning kernels over different domains.


Passing Tests without Memorizing: Two Models for Fooling Discriminators

arXiv.org Machine Learning

We introduce two mathematical frameworks for foolability in the context of generative distribution learning. In a nuthsell, fooling is an algorithmic task in which the input sample is drawn from some target distribution and the goal is to output a synthetic distribution that is indistinguishable from the target w.r.t to some fixed class of tests. This framework received considerable attention in the context of Generative Adversarial Networks (GANs), a recently proposed approach which achieves impressive empirical results. From a theoretical viewpoint this problem seems difficult to model. This is due to the fact that in its basic form, the notion of foolability is susceptible to a type of overfitting called memorizing. This raises a challenge of devising notions and definitions that separate between fooling algorithms that generate new synthetic data vs. algorithms that merely memorize or copy the training set. The first model we consider is called GAM--Foolability and is inspired by GANs. Here the learner has only an indirect access to the target distribution via a discriminator. The second model, called DP--Foolability, exploits the notion of differential privacy as a candidate criterion for non-memorization. We proceed to characterize foolability within these two models and study their interrelations. We show that DP--Foolability implies GAM--Foolability and prove partial results with respect to the converse. It remains, though, an open question whether GAM--Foolability implies DP--Foolability. We also present an application in the context of differentially private PAC learning. We show that from a statistical perspective, for any class H, learnability by a private proper learner is equivalent to the existence of a private sanitizer for H. This can be seen as an analogue of the equivalence between uniform convergence and learnability in classical PAC learning.