Total: 135

#1 Conference on Learning Theory 2021: Post-conference Preface [PDF] [Copy] [Kimi]

Authors: Mikhail Belkin ; Kpotufe Samory

No summary was provided.

#2 Stochastic block model entropy and broadcasting on trees with survey [PDF] [Copy] [Kimi]

Authors: Emmanuel Abbe ; Elisabetta Cornacchia ; Yuzhou Gu ; Yury Polyanskiy

The limit of the entropy in the stochastic block model (SBM) has been characterized in the sparse regime for the special case of disassortative communities [Coja-Oghlan et al. (2017)] and for the classical case of assortative communities but in the dense regime [Deshpande et al. (2016)]. The problem has not been closed in the classical sparse and assortative case. This paper establishes the result in this case for any SNR besides for the interval (1, 3.513). It further gives an approximation to the limit in this window. The result is obtained by expressing the global SBM entropy as an integral of local tree entropies in a broadcasting on tree model with erasure side-information. The main technical advancement then relies on showing the irrelevance of the boundary in such a model, also studied with variants in [Kanade et al. (2016)], [Mossel et al. (2016)] and [Mossel and Xu (2015)]. In particular, we establish the uniqueness of the BP fixed point in the survey model for any SNR above 3.513 or below 1. This only leaves a narrow region in the plane between SNR and survey strength where the uniqueness of BP conjectured in these papers remains unproved.

#3 Regret Minimization in Heavy-Tailed Bandits [PDF] [Copy] [Kimi1]

Authors: Shubhada Agrawal ; Sandeep K. Juneja ; Wouter M. Koolen

We revisit the classic regret-minimization problem in the stochastic multi-armed bandit setting when the arm-distributions are allowed to be heavy-tailed. Regret minimization has been well studied in simpler settings of either bounded support reward distributions or distributions that belong to a single parameter exponential family. We work under the much weaker assumption that the moments of order \((1+\epsilon)\){are} uniformly bounded by a known constant \(B\), for some given \( \epsilon > 0\). We propose an optimal algorithm that matches the lower bound exactly in the first-order term. We also give a finite-time bound on its regret. We show that our index concentrates faster than the well-known truncated or trimmed empirical mean estimators for the mean of heavy-tailed distributions. Computing our index can be computationally demanding. To address this, we develop a batch-based algorithm that is optimal up to a multiplicative constant depending on the batch size. We hence provide a controlled trade-off between statistical optimality and computational cost.

#4 SGD Generalizes Better Than GD (And Regularization Doesn’t Help) [PDF] [Copy] [Kimi]

Authors: Idan Amir ; Tomer Koren ; Roi Livni

We give a new separation result between the generalization performance of stochastic gradient descent (SGD) and of full-batch gradient descent (GD) in the fundamental stochastic convex optimization model. While for SGD it is well-known that $O(1/\epsilon^2)$ iterations suffice for obtaining a solution with $\epsilon$ excess expected risk, we show that with the same number of steps GD may overfit and emit a solution with $\Omega(1)$ generalization error. Moreover, we show that in fact $\Omega(1/\epsilon^4)$ iterations are necessary for GD to match the generalization performance of SGD, which is also tight due to recent work by Bassily et al. (2020). We further discuss how regularizing the empirical risk minimized by GD essentially does not change the above result, and revisit the concepts of stability, implicit bias and the role of the learning algorithm in generalization.

#5 The Bethe and Sinkhorn Permanents of Low Rank Matrices and Implications for Profile Maximum Likelihood [PDF] [Copy] [Kimi1]

Authors: Nima Anari ; Moses Charikar ; Kirankumar Shiragur ; Aaron Sidford

In this paper we consider the problem of computing the likelihood of the profile of a discrete distribution, i.e., the probability of observing the multiset of element frequencies, and computing a profile maximum likelihood (PML) distribution, i.e., a distribution with the maximum profile likelihood. For each problem we provide polynomial time algorithms that given $n$ i.i.d. samples from a discrete distribution, achieve an approximation factor of $\exp\left(-O(\sqrt{n} \log n) \right)$, improving upon the previous best-known bound achievable in polynomial time of $\exp(-O(n^{2/3} \log n))$ (Charikar, Shiragur and Sidford, 2019). Through the work of Acharya, Das, Orlitsky and Suresh (2016), this implies a polynomial time universal estimator for symmetric properties of discrete distributions in a broader range of error parameter. To obtain our results on PML we establish new connections between PML and the well-studied Bethe and Sinkhorn approximations to the permanent (Vontobel, 2012 and 2014). It is known that the PML objective is proportional to the permanent of a certain Vandermonde matrix (Vontobel, 2012) with $\sqrt{n}$ distinct columns, i.e. with non-negative rank at most $\sqrt{n}$. This allows us to show that the convex approximation to computing PML distributions studied in (Charikar, Shiragur and Sidford, 2019) is governed, in part, by the quality of Sinkhorn approximations to the permanent. We show that both Bethe and Sinkhorn permanents are $\exp(O(k \log(N/k)))$ approximations to the permanent of $N \times N$ matrices with non-negative rank at most $k$. This improves upon the previous known bounds of $\exp(O(N))$ and combining these insights with careful rounding of the convex relaxation yields our results.

#6 Learning in Matrix Games can be Arbitrarily Complex [PDF] [Copy] [Kimi1]

Authors: Gabriel P. Andrade ; Rafael Frongillo ; Georgios Piliouras

Many multi-agent systems with strategic interactions have their desired functionality encoded as the Nash equilibrium of a game, e.g. machine learning architectures such as Generative Adversarial Networks. Directly computing a Nash equilibrium of these games is often impractical or impossible in practice, which has led to the development of numerous learning algorithms with the goal of iteratively converging on a Nash equilibrium. Unfortunately, the dynamics generated by the learning process can be very intricate and instances failing to converge become hard to interpret. In this paper we show that, in a strong sense, this dynamic complexity is inherent to games. Specifically, we prove that replicator dynamics, the continuous-time analogue of Multiplicative Weights Update, even when applied in a very restricted class of games–known as finite matrix games–is rich enough to be able to approximate arbitrary dynamical systems. In the context of machine learning, our results are positive in the sense that they show the nearly boundless dynamic modelling capabilities of current machine learning practices, but also negative in implying that these capabilities may come at the cost of interpretability. As a concrete example, we show how replicator dynamics can effectively reproduce the well-known strange attractor of Lonrenz dynamics (the “butterfly effect") while achieving no regret.

#7 Functions with average smoothness: structure, algorithms, and learning [PDF] [Copy] [Kimi]

Authors: Yair Ashlagi ; Lee-Ad Gottlieb ; Aryeh Kontorovich

We initiate a program of average smoothness analysis for efficiently learning real-valued functions on metric spaces. Rather than using the Lipschitz constant as the regularizer, we define a local slope at each point and gauge the function complexity as the average of these values. Since the mean can be dramatically smaller than the maximum, this complexity measure can yield considerably sharper generalization bounds — assuming that these admit a refinement where the Lipschitz constant is replaced by our average of local slopes. In addition to the usual average, we also examine a “weak” average that is more forgiving and yields a much wider function class. Our first major contribution is to obtain just such distribution-sensitive bounds. This required overcoming a number of technical challenges, perhaps the most formidable of which was bounding the {\em empirical} covering numbers, which can be much worse-behaved than the ambient ones. Our combinatorial results are accompanied by efficient algorithms for smoothing the labels of the random sample, as well as guarantees that the extension from the sample to the whole space will continue to be, with high probability, smooth on average. Along the way we discover a surprisingly rich combinatorial and analytic structure in the function class we define.

#8 Adversarially Robust Low Dimensional Representations [PDF] [Copy] [Kimi1]

Authors: Pranjal Awasthi ; Vaggos Chatziafratis ; Xue Chen ; Aravindan Vijayaraghavan

Many machine learning systems are vulnerable to small perturbations made to inputs either at test time or at training time. This has received much recent interest on the empirical front due to applications where reliability and security are critical. However, theoretical understanding of algorithms that are robust to adversarial perturbations is limited. In this work we focus on Principal Component Analysis (PCA), a ubiquitous algorithmic primitive in machine learning. We formulate a natural robust variant of PCA where the goal is to find a low dimensional subspace to represent the given data with minimum projection error, that is in addition robust to small perturbations measured in $\ell_q$ norm (say $q=\infty$). Unlike PCA which is solvable in polynomial time, our formulation is computationally intractable to optimize as it captures a variant of the well-studied sparse PCA objective as a special case. We show the following results: 1. Polynomial time algorithm that is constant factor competitive in the worst-case with respect to the best subspace, in terms of the projection error and the robustness criterion. 2. We show that our algorithmic techniques can also be made robust to adversarial training-time perturbations, in addition to yielding representations that are robust to adversarial perturbations at test time. Specifically, we design algorithms for a strong notion of training-time perturbations, where every point is adversarially perturbed up to a specified amount. 3. We illustrate the broad applicability of our algorithmic techniques in addressing robustness to adversarial perturbations, both at training time and test time. In particular, our adversarially robust PCA primitive leads to computationally efficient and robust algorithms for both unsupervised and supervised learning problems such as clustering and learning adversarially robust classifiers.

#9 The Last-Iterate Convergence Rate of Optimistic Mirror Descent in Stochastic Variational Inequalities [PDF] [Copy] [Kimi1]

Authors: Waïss Azizian ; Franck Iutzeler ; Jérôme Malick ; Panayotis Mertikopoulos

In this paper, we analyze the local convergence rate of optimistic mirror descent methods in stochastic variational inequalities, a class of optimization problems with important applications to learning theory and machine learning. Our analysis reveals an intricate relation between the algorithm’s rate of convergence and the local geometry induced by the method’s underlying Bregman function. We quantify this relation by means of the Legendre exponent, a notion that we introduce to measure the growth rate of the Bregman divergence relative to the ambient norm near a solution. We show that this exponent determines both the optimal step-size policy of the algorithm and the optimal rates attained, explaining in this way the differences observed for some popular Bregman functions (Euclidean projection, negative entropy, fractional power, etc.).

#10 Optimal Dynamic Regret in Exp-Concave Online Learning [PDF] [Copy] [Kimi]

Authors: Dheeraj Baby ; Yu-Xiang Wang

We consider the problem of the Zinkevich (2003)-style dynamic regret minimization in online learning with \emph{exp-concave} losses. We show that whenever improper learning is allowed, a Strongly Adaptive online learner achieves the dynamic regret of $\tilde O^*(n^{1/3}C_n^{2/3} \vee 1)$ where $C_n$ is the \emph{total variation} (a.k.a. \emph{path length}) of the an arbitrary sequence of comparators that may not be known to the learner ahead of time. Achieving this rate was highly nontrivial even for square losses in 1D where the best known upper bound was $O(\sqrt{nC_n} \vee \log n)$ (Yuan and Lamperski, 2019). Our new proof techniques make elegant use of the intricate structures of the primal and dual variables imposed by the KKT conditions and could be of independent interest. Finally, we apply our results to the classical statistical problem of \emph{locally adaptive non-parametric regression} (Mammen, 1991; Donoho and Johnstone, 1998) and obtain a stronger and more flexible algorithm that do not require any statistical assumptions or any hyperparameter tuning.

#11 Spectral Planting and the Hardness of Refuting Cuts, Colorability, and Communities in Random Graphs [PDF] [Copy] [Kimi1]

Authors: Afonso S Bandeira ; Jess Banks ; Dmitriy Kunisky ; Christopher Moore ; Alex Wein

We study the problem of efficiently refuting the k-colorability of a graph, or equivalently, certifying a lower bound on its chromatic number. We give formal evidence of average-case computational hardness for this problem in sparse random regular graphs, suggesting that there is no polynomial-time algorithm that improves upon a classical spectral algorithm. Our evidence takes the form of a "computationally-quiet planting": we construct a distribution of d-regular graphs that has significantly smaller chromatic number than a typical regular graph drawn uniformly at random, while providing evidence that these two distributions are indistinguishable by a large class of algorithms. We generalize our results to the more general problem of certifying an upper bound on the maximum k-cut. This quiet planting is achieved by minimizing the effect of the planted structure (e.g. colorings or cuts) on the graph spectrum. Specifically, the planted structure corresponds exactly to eigenvectors of the adjacency matrix. This avoids the pushout effect of random matrix theory, and delays the point at which the planting becomes visible in the spectrum or local statistics. To illustrate this further, we give similar results for a Gaussian analogue of this problem: a quiet version of the spiked model, where we plant an eigenspace rather than adding a generic low-rank perturbation. Our evidence for computational hardness of distinguishing two distributions is based on three different heuristics: stability of belief propagation, the local statistics hierarchy, and the low-degree likelihood ratio. Of independent interest, our results include general-purpose bounds on the low-degree likelihood ratio for multi-spiked matrix models, and an improved low-degree analysis of the stochastic block model.

#12 Non-Euclidean Differentially Private Stochastic Convex Optimization [PDF] [Copy] [Kimi1]

Authors: Raef Bassily ; Cristobal Guzman ; Anupama Nandi

Differentially private (DP) stochastic convex optimization (SCO) is a fundamental problem, where the goal is to approximately minimize the population risk with respect to a convex loss function, given a dataset of i.i.d. samples from a distribution, while satisfying differential privacy with respect to the dataset. Most of the existing works in the literature of private convex optimization focus on the Euclidean (i.e., $\ell_2$) setting, where the loss is assumed to be Lipschitz (and possibly smooth) w.r.t. the $\ell_2$ norm over a constraint set with bounded $\ell_2$ diameter. Algorithms based on noisy stochastic gradient descent (SGD) are known to attain the optimal excess risk in this setting. In this work, we conduct a systematic study of DP-SCO for $\ell_p$-setups. For $p=1$, under a standard smoothness assumption, we give a new algorithm with nearly optimal excess risk. This result also extends to general polyhedral norms and feasible sets. For $p\in(1, 2)$, we give two new algorithms, whose central building block is a novel privacy mechanism, which generalizes the Gaussian mechanism. Moreover, we establish a lower bound on the excess risk for this range of $p$, showing a necessary dependence on $\sqrt{d}$, where $d$ is the dimension of the space. Our lower bound implies a sudden transition of the excess risk at $p=1$, where the dependence on $d$ changes from logarithmic to polynomial, resolving an open question in prior work \citep{TTZ15a}. For $p\in (2, \infty)$, noisy SGD attains optimal excess risk in the low-dimensional regime; in particular, this proves the optimality of noisy SGD for $p=\infty$. Our work draws upon concepts from the geometry of normed spaces, such as the notions of regularity, uniform convexity, and uniform smoothness.

#13 Reconstructing weighted voting schemes from partial information about their power indices [PDF] [Copy] [Kimi]

Authors: Huck Bennett ; Anindya De ; Rocco Servedio ; Emmanouil Vasileios Vlatakis-Gkaragkounis

A number of recent works [Goldberg 2006; O’Donnell and Servedio 2011; De, Diakonikolas, and Servedio 2017; De, Diakonikolas, Feldman, and Servedio 2014] have considered the problem of approximately reconstructing an unknown weighted voting scheme given information about various sorts of “power indices” that characterize the level of control that individual voters have over the final outcome. In the language of theoretical computer science, this is the problem of approximating an unknown linear threshold function (LTF) over ${-1,1}^n$ given some numerical measure (such as the function’s n “Chow parameters,” a.k.a. its degree-1 Fourier coefficients, or the vector of its n Shapley indices) of how much each of the n individual input variables affects the outcome of the function. In this paper we consider the problem of reconstructing an LTF given only partial information about its Chow parameters or Shapley indices; i.e. we are given only the Chow parameters or the Shapley indices corresponding to a subset $S\subseteq [n]$ of the n input variables. A natural goal in this partial information setting is to find an LTF whose Chow parameters or Shapley indices corresponding to indices in S accurately match the given Chow parameters or Shapley indices of the unknown LTF. We refer to this as the Partial Inverse Power Index Problem. Our main results are a polynomial time algorithm for the ($\epsilon$-approximate) Chow Parameters Partial Inverse Power Index Problem and a quasi-polynomial time algorithm for the ($\epsilon$-approximate) Shapley Indices Partial Inverse Power Index Problem.

#14 Deterministic Finite-Memory Bias Estimation [PDF] [Copy] [Kimi]

Authors: Tomer Berg ; Or Ordentlich ; Ofer Shayevitz

In this paper we consider the problem of estimating a Bernoulli parameter using finite memory. Let $X_1,X_2,\ldots$ be a sequence of independent identically distributed Bernoulli random variables with expectation $\theta$, where $\theta \in [0,1]$. Consider a finite-memory deterministic machine with $S$ states, that updates its state $M_n \in \{1,2,\ldots,S\}$ at each time according to the rule $M_n = f(M_{n-1},X_n)$, where $f$ is a deterministic time-invariant function. Assume that the machine outputs an estimate at each time point according to some fixed mapping from the state space to the unit interval. The quality of the estimation procedure is measured by the asymptotic risk, which is the long-term average of the instantaneous quadratic risk. The main contribution of this paper is an upper bound on the smallest worst-case asymptotic risk any such machine can attain. This bound coincides with a lower bound derived by Leighton and Rivest, to imply that $\Theta(1/S)$ is the minimax asymptotic risk for deterministic $S$-state machines. In particular, our result disproves a longstanding $\Theta(\log S/S)$ conjecture for this quantity, also posed by Leighton and Rivest.

#15 Online Learning from Optimal Actions [PDF] [Copy] [Kimi]

Authors: Omar Besbes ; Yuri Fonseca ; Ilan Lobel

We study the problem of online contextual optimization where, at each period, instead of observing the loss, we observe, after-the-fact, the optimal action an oracle with full knowledge of the objective function would have taken. At each period, the decision-maker has access to a new set of feasible actions to select from and to a new contextual function that affects that period’s loss function. We aim to minimize regret, which is defined as the difference between our losses and the ones incurred by an all-knowing oracle. We obtain the first regret bound for this problem that is logarithmic in the time horizon. Our results are derived through the development and analysis of a novel algorithmic structure that leverages the underlying geometry of the problem.

#16 Majorizing Measures, Sequential Complexities, and Online Learning [PDF] [Copy] [Kimi]

Authors: Adam Block ; Yuval Dagan ; Alexander Rakhlin

We introduce the technique of generic chaining and majorizing measures for controlling sequential Rademacher complexity. We relate majorizing measures to the notion of fractional covering numbers, which we show to be dominated in terms of sequential scale-sensitive dimensions in a horizon-independent way, and, under additional complexity assumptions establish a tight control on worst-case sequential Rademacher complexity in terms of the integral of sequential scale-sensitive dimension. Finally, we establish a tight contraction inequality for worst-case sequential Rademacher complexity. The above constitutes the resolution of a number of outstanding open problems in extending the classical theory of empirical processes to the sequential case, and, in turn, establishes sharp results for online learning.

#17 Robust learning under clean-label attack [PDF] [Copy] [Kimi]

Authors: Avrim Blum ; Steve Hanneke ; Jian Qian ; Han Shao

We study the problem of robust learning under clean-label data-poisoning attacks, where the attacker injects (an arbitrary set of) \emph{correctly-labeled} examples to the training set to fool the algorithm into making mistakes on \emph{specific} test instances at test time. The learning goal is to minimize the attackable rate (the probability mass of attackable test instances), which is more difficult than optimal PAC learning. As we show, any robust algorithm with diminishing attackable rate can achieve the optimal dependence on $\epsilon$ in its PAC sample complexity, i.e., $O(1/\epsilon)$. On the other hand, the attackable rate might be large even for some optimal PAC learners, e.g., SVM for linear classifiers. Furthermore, we show that the class of linear hypotheses is not robustly learnable when the data distribution has zero margin and is robustly learnable in the case of positive margin but requires sample complexity exponential in the dimension. For a general hypothesis class with bounded VC dimension, if the attacker is limited to add at most $t=O(1/\epsilon)$ poison examples, the optimal robust learning sample complexity grows linearly with $t$.

#18 Multiplayer Bandit Learning, from Competition to Cooperation [PDF] [Copy] [Kimi]

Authors: Simina Branzei ; Yuval Peres

The stochastic multi-armed bandit model captures the tradeoff between exploration and exploitation. We study the effects of competition and cooperation on this tradeoff. Suppose there are two arms, one predictable and one risky, and two players, Alice and Bob. In every round, each player pulls an arm, receives the resulting reward, and observes the choice of the other player but not their reward. Alice’s utility is $\Gamma_A + \lambda \Gamma_B$ (and similarly for Bob), where $\Gamma_A$ is Alice’s total reward and $\lambda \in [-1, 1]$ is a cooperation parameter. At $\lambda = -1$ the players are competing in a zero-sum game, at $\lambda = 1$, their interests are aligned, and at $\lambda = 0$, they are neutral: each player’s utility is their own reward. The model is related to the economics literature on strategic experimentation, where usually players observe each other’s rewards. Suppose the predictable arm has success probability $p$ and the risky arm has prior $\mu$. If the discount factor is $\beta$, then the value of $p$ where a single player is indifferent between the arms is the Gittins index $g = g(\mu,\beta) > m$, where $m$ is the mean of the risky arm. Our first result answers, in this setting, a fundamental question posed by Rothschild \cite{rotschild}. We show that competing and neutral players eventually settle on the same arm (even though it may not be the best arm) in every Nash equilibrium, while this can fail for players with aligned interests. Moreover, we show that \emph{competing players} explore \emph{less} than a single player: there is $p^* \in (m, g)$ so that for all $p > p^*$, the players stay at the predictable arm. However, the players are not myopic: they still explore for some $p > m$. On the other hand, \emph{cooperating players} (with $\lambda =1$) explore \emph{more} than a single player. We also show that \emph{neutral players} learn from each other, receiving strictly higher total rewards than they would playing alone, for all $ p\in (p^*, g)$, where $p^*$ is the threshold above which competing players do not explore.

#19 Near Optimal Distributed Learning of Halfspaces with Two Parties [PDF] [Copy] [Kimi]

Authors: Mark Braverman ; Gillat Kol ; Shay Moran ; Raghuvansh R. Saxena

<i>Distributed learning</i> protocols are designed to train on distributed data without gathering it all on a single centralized machine, thus contributing to the efficiency of the system and enhancing its privacy. We study a central problem in distributed learning, called {\it distributed learning of halfspaces}: let $U \subseteq \mathbb{R}^d$ be a known domain of size $n$ and let $h:\mathbb{R}^d\to \mathbb{R}$ be an unknown target affine function.\footnote{In practice, the domain $U$ is defined implicitly by the representation of $d$-dimensional vectors which is used in the protocol.} A set of <i>examples</i> $\{(u,b)\}$ is distributed between several parties, where~$u \in U$ is a point and $b = \mathsf{sign}(h(u)) \in \{\pm 1\}$ is its label. The parties goal is to agree on a classifier~$f: U\to\{\pm 1\}$ such that~$f(u)=b$ for every input example~$(u,b)$. We design a protocol for the distributed halfspace learning problem in the two-party setting, communicating only $\tilde O(d\log n)$ bits. To this end, we introduce a new tool called <i>halfspace containers</i>, that is closely related to <i>bracketing numbers</i> in statistics and to <i>hyperplane cuttings</i> in discrete geometry, and allows for a compressed approximate representation of every halfspace. We complement our upper bound result by an almost matching $\tilde \Omega(d\log n)$ lower bound on the communication complexity of any such protocol Since the distributed halfspace learning problem is closely related to the <i>convex set disjointness</i> problem in communication complexity and the problem of <i>distributed linear programming</i> in distributed optimization, we also derive upper and lower bounds of $\tilde O(d^2\log n)$ and~$\tilde{\Omega}(d\log n)$ on the communication complexity of both of these basic problems.

#20 Near-Optimal Entrywise Sampling of Numerically Sparse Matrices [PDF] [Copy] [Kimi]

Authors: Vladimir Braverman ; Robert Krauthgamer ; Aditya R. Krishnan ; Shay Sapir

Many real-world data sets are sparse or almost sparse. One method to measure this for a matrix $A\in \mathbb{R}^{n\times n}$ is the \emph{numerical sparsity}, denoted $\mathsf{ns}(A)$, defined as the minimum $k\geq 1$ such that $\|a\|_1/\|a\|_2 \leq \sqrt{k}$ for every row and every column $a$ of $A$. This measure of $a$ is smooth and is clearly only smaller than the number of non-zeros in the row/column $a$. The seminal work of Achlioptas and McSherry (2007) has put forward the question of approximating an input matrix $A$ by entrywise sampling. More precisely, the goal is to quickly compute a sparse matrix $\tilde{A}$ satisfying $\|A - \tilde{A}\|_2 \leq \epsilon \|A\|_2$ (i.e., additive spectral approximation) given an error parameter $\epsilon>0$. The known schemes sample and rescale a small fraction of entries from $A$. We propose a scheme that sparsifies an almost-sparse matrix $A$ — it produces a matrix $\tilde{A}$ with $O(\epsilon^{-2}\mathsf{ns}(A) \cdot n\ln n)$ non-zero entries with high probability. We also prove that this upper bound on $\mathsf{nnz}(\tilde{A})$ is \emph{tight} up to logarithmic factors. Moreover, our upper bound improves when the spectrum of $A$ decays quickly (roughly replacing $n$ with the stable rank of $A$). Our scheme can be implemented in time $O(\mathsf{nnz}(A))$ when $\|A\|_2$ is given. Previously, a similar upper bound was obtained by Achlioptas et al. (2013), but only for a restricted class of inputs that does not even include symmetric or covariance matrices. Finally, we demonstrate two applications of these sampling techniques, to faster approximate matrix multiplication, and to ridge regression by using sparse preconditioners.

#21 Statistical Query Algorithms and Low Degree Tests Are Almost Equivalent [PDF] [Copy] [Kimi]

Authors: Matthew S Brennan ; Guy Bresler ; Sam Hopkins ; Jerry Li ; Tselil Schramm

Researchers currently use a number of approaches to predict and substantiate information-computation gaps in high-dimensional statistical estimation problems. A prominent approach is to characterize the limits of restricted models of computation, which on the one hand yields strong computational lower bounds for powerful classes of algorithms and on the other hand helps guide the development of efficient algorithms. In this paper, we study two of the most popular restricted computational models, the statistical query framework and low-degree polynomials, in the context of high-dimensional hypothesis testing. Our main result is that under mild conditions on the testing problem, the two classes of algorithms are essentially equivalent in power. As corollaries, we obtain new statistical query lower bounds for sparse PCA, tensor PCA and several variants of the planted clique problem.

#22 Exact Recovery of Clusters in Finite Metric Spaces Using Oracle Queries [PDF] [Copy] [Kimi]

Authors: Marco Bressan ; Nicoló Cesa-Bianchi ; Silvio Lattanzi ; Andrea Paudice

We investigate the problem of exact cluster recovery using oracle queries. Previous results show that clusters in Euclidean spaces that are convex and separated with a margin can be reconstructed exactly using only $O(\log n)$ same-cluster queries, where $n$ is the number of input points. In this work, we study this problem in the more challenging non-convex setting. We introduce a structural characterization of clusters, called $(\beta,\gamma)$-convexity, that can be applied to any finite set of points equipped with a metric (or even a semimetric, as the triangle inequality is not needed). Using $(\beta,\gamma)$-convexity, we can translate natural density properties of clusters (which include, for instance, clusters that are strongly non-convex in $R^d$) into a graph-theoretic notion of convexity. By exploiting this convexity notion, we design a deterministic algorithm that recovers $(\beta,\gamma)$-convex clusters using $O(k^2 \log n + k^2 (\frac{6}{\beta\gamma})^{dens(X)})$ same-cluster queries, where $k$ is the number of clusters and $dens(X)$ is the density dimension of the semimetric. We show that an exponential dependence on the density dimension is necessary, and we also show that, if we are allowed to make $O(k^2 + k \log n)$ additional queries to a "cluster separation" oracle, then we can recover clusters that have different and arbitrary scales, even when the scale of each cluster is unknown.

#23 A Law of Robustness for Two-Layers Neural Networks [PDF] [Copy] [Kimi]

Authors: Sebastien Bubeck ; Yuanzhi Li ; Dheeraj M Nagaraj

We initiate the study of the inherent tradeoffs between the size of a neural network and its robustness, as measured by its Lipschitz constant. We make a precise conjecture that, for any Lipschitz activation function and for most datasets, any two-layers neural network with $k$ neurons that perfectly fit the data must have its Lipschitz constant larger (up to a constant) than $\sqrt{n/k}$ where $n$ is the number of datapoints. In particular, this conjecture implies that overparametrization is necessary for robustness, since it means that one needs roughly one neuron per datapoint to ensure a $O(1)$-Lipschitz network, while mere data fitting of $d$-dimensional data requires only one neuron per $d$ datapoints. We prove a weaker version of this conjecture when the Lipschitz constant is replaced by an upper bound on it based on the spectral norm of the weight matrix. We also prove the conjecture in the high-dimensional regime $n \approx d$ (which we also refer to as the undercomplete case, since only $k \leq d$ is relevant here). Finally we prove the conjecture for polynomial activation functions of degree $p$ when $n \approx d^p$. We complement these findings with experimental evidence supporting the conjecture.

#24 Cooperative and Stochastic Multi-Player Multi-Armed Bandit: Optimal Regret With Neither Communication Nor Collisions [PDF] [Copy] [Kimi]

Authors: Sebastien Bubeck ; Thomas Budzinski ; Mark Sellke

We consider the cooperative multi-player version of the stochastic multi-armed bandit problem. We study the regime where the players cannot communicate but have access to shared randomness. In prior work by the first two authors, a strategy for this regime was constructed for two players and three arms, with regret $\tilde{O}(\sqrt{T})$, and with no collisions at all between the players (with very high probability). In this paper we show that these properties (near-optimal regret and no collisions at all) are achievable for any number of players and arms. At a high level, the previous strategy heavily relied on a 2-dimensional geometric intuition that was difficult to generalize in higher dimensions, while here we take a more combinatorial route to build the new strategy.

#25 Fast Rates for Structured Prediction [PDF] [Copy] [Kimi]

Authors: Vivien A Cabannes ; Francis Bach ; Alessandro Rudi

Discrete supervised learning problems such as classification are often tackled by introducing a continuous surrogate problem akin to regression. Bounding the original error, between estimate and solution, by the surrogate error endows discrete problems with convergence rates already shown for continuous instances. Yet, current approaches do not leverage the fact that discrete problems are essentially predicting a discrete output when continuous problems are predicting a continuous value. In this paper, we tackle this issue for general structured prediction problems, opening the way to “super fast” rates, that is, convergence rates for the excess risk faster than $n^{-1}$, where $n$ is the number of observations, with even exponential rates with the strongest assumptions. We first illustrate it for predictors based on nearest neighbors, generalizing rates known for binary classification to any discrete problem within the framework of structured prediction. We then consider kernel ridge regression where we improve known rates in $n^{-1/4}$ to arbitrarily fast rates, depending on a parameter characterizing the hardness of the problem, thus allowing, under smoothness assumptions, to bypass the curse of dimensionality.