Amos Fiat

Tel Aviv University | TAU · Department of Computer Science

We study combinatorial auctions with interdependent valuations, where each agent i has a private signal s i that captures her private information and the valuation function of every agent depends on the entire signal profile, [Formula: see text]. The literature in economics shows that the interdependent model gives rise to strong impossibility resu...

The existence of EFX allocations of goods is a major open problem in fair division, even for additive valuations. The current state of the art is that no setting where EFX allocations are impossible is known, and yet, existence results are known only for very restricted settings, such as: (i) agents with identical valuations, (ii) 2 agents, and (ii...

We consider a market setting of agents with additive valuations over heterogeneous divisible resources. Agents are assigned a budget of tokens (possibly unequal budgets) they can use to obtain resources; leftover tokens are worthless. We show how to support any Pareto efficient allocation in equilibrium, using anonymous resource prices and agent sp...

The existence of EFX allocations is a major open problem in fair division, even for additive valuations. The current state of the art is that no setting where EFX allocations are impossible is known, and EFX is known to exist for ($i$) agents with identical valuations, ($ii$) 2 agents, ($iii$) 3 agents with additive valuations, ($iv$) agents with o...

The Bitcoin protocol induces miners, through monetary rewards, to expend energy in order to add blocks to the chain. We show that, when energy costs are substantial and taken into account, counterintuitive and unintended strategic behavior results: In a simple bounded-horizon setting with two identical miners there is a unique pure symmetric equili...

We study combinatorial auctions with interdependent valuations. In such settings, each agent $i$ has a private signal $s_i$ that captures her private information, and the valuation function of every agent depends on the entire signal profile, ${\bs}=(s_1,\ldots,s_n)$. Previous results in economics concentrated on identifying (often stringent) assum...

The Nash equilibrium as a prediction myopically ignores the possibility that deviating from the equilibrium could lead to an avalanche of beneficial changes by other agents. We consider a non-myopic version of Cournot competition, where each firm selects either profit maximization (as in the classical model) or revenue maximization (by masquerading...

We consider a setting where an auctioneer sells a single item to $n$ potential agents with {\em interdependent values}. That is, each agent has her own private signal, and the valuation of each agent is a known function of all $n$ private signals. This captures settings such as valuations for artwork, oil drilling rights, broadcast rights, and many...

We consider a setting where an auctioneer sells a single item to n potential agents with interdependent values. That is, each agent has her own private signal, and the valuation of each agent is a known function of all n private signals. This captures settings such as valuations for oil drilling rights, broadcast rights, pieces of art, and many mor...

We give a simple proof showing that the RANKING algorithm introduced by Karp, Vazirani and Vazirani \cite{DBLP:conf/stoc/KarpVV90} is $1-\frac{1}{e}$ competitive for the online bipartite matching problem. Our proof resembles the proof given by Devanur, Jain and Kleinberg [2013], but does not make an explicit use of linear programming duality; inste...

We give a prompt online mechanism for minimizing the sum of [weighted] completion times. This is the first prompt online algorithm for the problem. When such jobs are strategic agents, delaying scheduling decisions makes little sense. Moreover, the mechanism has a particularly simple form of an anonymous menu of options.

We consider job scheduling settings, with multiple machines, where jobs arrive online and choose a machine selfishly so as to minimize their cost. Our objective is the classic makespan minimization objective, which corresponds to the completion time of the last job to complete. The incentives of the selfish jobs may lead to poor performance. To rec...

We consider job scheduling settings, with multiple machines, where jobs arrive online and choose a machine selfishly so as to minimize their cost. Our objective is the classic makespan minimization objective, which corresponds to the completion time of the last job to complete. The incentives of the selfish jobs may lead to poor performance. To rec...

We consider the online carpool fairness problem of Fagin–Williams (IBM J Res Dev 27(2):133–139, 1983), where an online algorithm is presented with a sequence of pairs drawn from a group of n potential drivers. The online algorithm must select one driver from each pair, with the objective of partitioning the driving burden as fairly as possible for...

Computing driving directions has motivated many shortest path algorithms based on preprocessing. Given a graph, the preprocessing stage computes a modest amount of auxiliary data, which is then used to speed up online queries. In practice, the best algorithms have storage overhead comparable to the graph size and answer queries very fast, while exa...

How should we evaluate a rumor? We address this question in a setting where multiple agents seek an estimate of the probability, b, of some future binary event. A common uniform prior on b is assumed. A rumor about b meanders through the network, evolving over time. The rumor evolves, not because of ill will or noise, but because agents incorporate...

We study mechanisms for candidate selection that seek to minimize the social cost, where voters and candidates are associated with points in some underlying metric space. The social cost of a candidate is the sum of its distances to each voter. Some of our work assumes that these points can be modeled on the real line, but other results of ours are...

Walrasian prices, if they exist, have the property that one can assign every buyer some bundle in her demand set, such that the resulting assignment will maximize social welfare. Unfortunately, this assumes carefully breaking ties amongst different bundles in the buyer demand set. Presumably, the shopkeeper cleverly convinces the buyer to break tie...

We extend the notion of Combinatorial Walrasian Equilibrium, as defined by \citet{FGL13}, to settings with budgets. When agents have budgets, the maximum social welfare as traditionally defined is not a suitable benchmark since it is overly optimistic. This motivated the liquid welfare of \cite{DP14} as an alternative. Observing that no combinatori...

Consider the pricing problem faced by FedEx. Each customer has a package to ship, a deadline $d$ by which he needs his package to arrive, and a value $v$ for a guarantee that the package will arrive by his deadline. FedEx can (and does) offer a number of different shipping options in order to extract more revenue from their customers. In this paper...

In this paper we expand the standard Hotelling-Downs model of spatial competition to a setting where clients do not necessarily choose their closest candidate (retail product or political). Specifically, we consider a setting where clients may disavow all candidates if there is no candidate that is sufficiently close to the client preferences. More...

We study mechanisms for candidate selection that seek to minimize the social cost, where voters and candidates are associated with points in some underlying metric space. The social cost of a candidate is the sum of its distances to each voter. Some of our work assumes that these points can be modeled on a real line, but other results of ours are m...

Walrasian prices, if they exist, have the property that one can assign every buyer some bundle in her demand set, such that the resulting assignment will maximize social welfare. Unfortunately, this assumes carefully breaking ties amongst different bundles in the buyer demand set. Presumably, the shopkeeper cleverly convinces the buyer to break tie...

We consider a generalization of the secretary problem where contracts are temporary, and for a fixed duration. This models online hiring of temporary employees, or online auctions for re-usable resources. The problem is related to the question of Finding a large independent set in a random unit interval graph.

We consider dynamic pricing schemes in online settings where selfish agents generate online events. Previous work on online mechanisms has dealt almost entirely with the goal of maximizing social welfare or revenue in an auction settings. This paper deals with quite general settings and minimizing social costs. We show that appropriately computed p...

We give a new predecessor data structure which improves upon the index size of the Pǎtraşcu-Thorup data structures, reducing the index size from O(nw4/5)O(nw4/5) bits to O(nlog⁡w)O(nlog⁡w) bits, with optimal probe complexity. Alternatively, our new data structure can be viewed as matching the space complexity of the (probe-suboptimal) z -fast trie...

In the \emph{incremental cycle detection} problem arcs are added to a directed acyclic graph and the algorithm has to report if the new arc closes a cycle. One seeks to minimize the total time to process the entire sequence of arc insertions, or until a cycle appears. In a recent breakthrough, Bender, Fineman, Gilbert and Tarjan \cite{BeFiGiTa11} p...

Rackoff and Simon proved that a variant of Chaum’s protocol for anonymous communication, later developed as the Onion Routing Protocol, is unlinkable against a passive adversary that controls all communication links and most of the nodes in a communication system. A major drawback of their analysis is that the protocol is secure only if (almost) al...

We give a new successor data structure which improves upon the index size of the Pǎtraşcu-Thorup data structures, reducing the index size from O(n w 4/5) bits to O(n logw) bits, with optimal probe complexity. Alternatively, our new data structure can be viewed as matching the space complexity of the (probe-suboptimal) z-fast trie of Belazzougui et...

This paper introduces HLDB, the first practical system that can answer exact spatial queries on continental road networks entirely within a database. HLDB is based on hub labels (HL), the fastest point-to-point algorithm for road networks, and its queries are implemented (quite naturally) in standard SQL. Within the database, HLDB answers exact dis...

We introduce and study strongly truthful mechanisms and their applications. We use strongly truthful mechanisms as a tool for implementation in undominated strategies for several problems,including the design of externality resistant auctions and a variant of multi-dimensional scheduling.

The Communications Web site, http://cacm.acm.org, features more than a dozen bloggers in the BLOG@CACM community. In each issue of Communications, we'll publish selected posts or excerpts.twitterFollow us on Twitter ...

We study envy-free (EF) mechanisms for multi-unit auctions with budgeted agents that approximately maximize revenue. In an EF auction, prices are set so that every bidder receives a bundle that maximizes her utility amongst all bundles; We show that the problem of revenue-maximizing EF auctions is NP-hard, even for the case of identical items and a...

The Nash Equilibrium as a prediction myopically ignores the possibility that deviating from the equilibrium could lead to an avalanche of beneficial changes by other agents. We consider a non-myopic version of Cournot competition, where each firm selects either profit maximization (as in the classical model) or revenue maximization (by masquerading...

In this work we give a tight lower bound on makespan approximations for envy-free allocation mechanism dedicated to scheduling tasks on unrelated machines. Specifically, we show that no mechanism exists that can guarantee an envy-free allocation of jobs to $m$ machines with a makespan of less than a factor of $O(\log m)$ of the minimal makespan. Co...

We give a non-trivial class of valuation functions for which we give auctions that are efficient, truthful and envy-free. We give interesting classes of valuations for which one can design such auctions. Surprisingly, we also show that minor modifications to these valuations lead to impossibility results, the most surprising of which is that for a...

We consider the multivariate interlace polynomial introduced by Courcelle (Electron. J. Comb. 15(1), 2008), which generalizes several interlace polynomials defined by Arratia, Bollobás, and Sorkin (J. Comb. Theory Ser. B 92(2):199–233, ...

We consider budget constrained combinatorial auctions where each bidder has a private value for each of the items in some subset of the items and an overall budget constraint. Such auctions capture adword auctions, where advertisers offer a bid for those adwords that (hopefully) target their intended audience, and advertisers also have budgets. It...

Much of Game Theory, including the Nash equilibrium concept, is based on the assumption that players are expectation maximizers. It is known that if players are risk averse, games may no longer have Nash equilibria [11,6]. We show that 1 Under risk aversion (convex risk valuations), and for almost all games, there are no mixed Nash equilibria, and...

We study envy-free mechanisms for scheduling tasks on unrelated machines (agents) that approximately minimize the makespan. For indivisible tasks, we put forward an envy-free poly-time mechanism that approximates the minimal makespan to within a factor of O(log m), where m is the number of machines. We also show a lower bound of γ(log m / log log m...

We study auctions with additive valuations where agents have a limit on the number of goods they may receive. We refer to such valuations as {\em capacitated} and seek mechanisms that maximize social welfare and are simultaneously incentive compatible, envy-free, individually rational, and have no positive transfers. If capacities are infinite, the...

We study mechanisms for an allocation of goods among agents, where agents have no incentive to lie about their true values (incentive compatible) and for which no agent will seek to exchange outcomes with another (envy-free). Mechanisms satisfying each requirement separately have been studied extensively, but there are few results on mechanisms ach...

Computing driving directions has motivated many shortest path heuristics that answer queries on continental scale networks, with tens of millions of intersections, literally instantly, and with very low storage overhead. In this paper we complement the experimental evidence with the first rigorous proofs of efficiency for many of the heuristics sug...

We consider budget constrained combinatorial auctions where bidder $i$ has a private value $v_i$, a budget $b_i$, and is interested in all the items in $S_i$. The value to agent $i$ of a set of items $R$ is $|R \cap S_i| \cdot v_i$. Such auctions capture adword auctions, where advertisers offer a bid for ads in response to an advertiser-dependent s...

We study the envy free pricing problem faced by a seller who wishes to maximize revenue by setting prices for bundles of items. If there is an unlimited supply of items and agents are single minded then we show that finding the revenue maximizing envy free allocation/pricing can be solved in polynomial time by reducing it to an instance of weighted...

We study envy-free mechanisms for scheduling tasks on unrelated machines (agents) that approximately minimize the makespan. For indivisible tasks, we put forward an envy-free poly-time mechanism that approximates the minimal makespan to within a factor of $O(\log m)$, where $m$ is the number of machines. We also show a lower bound of $\Omega(\log m...

A coreset of a point set P is a small weighted set of points that captures some geometric properties of $P$. Coresets have found use in a vast host of geometric settings. We forge a link between coresets, and differentially private sanitizations that can answer any number of queries without compromising privacy. We define the notion of private core...

Digital rights management systems seek to control the use of proprietary material (e.g., copyrighted movies). The use of DRM introduces a new set of caching issues not previously studied. Technically, these problems include elements of ski rental algorithms as well as paging, and their generalizations, online “capital investment” and generalized ca...

We study selfish agents that have a “distorted view” of reality. We introduce a framework of subjective vs. objective reality. This is very useful to model risk averse behavior. Natural quality of service issues can be cast as special cases thereof. In particular, we study two applicable variants of the price of anarchy paradigm, the subjective pr...

We consider the online problem of non-preemptive queue management. An online sequence of packets arrive, each of which has an associated intrinsic value. Packets can be accepted to a FIFO queue, or discarded. The profit gained by transmitting a packet diminishes over time and is equal to its value minus the delay. This corresponds to the well known...

As defined by Aumann in 1959, a strong equilibrium is a Nash equilibrium that is resilient to deviations by coalitions. We give tight bounds on the strong price of anarchy for load balancing on related machines. We also give tight bounds for $k$-strong equilibria, where the size of a deviating coalition is at most $k$, for unrelated machines. @InPr...

The success of a P2P file-sharing network highly depends on the scalability and versatility of its search mechanism. Two particularly desirable search features are scope (ability to find infrequent items) and support for partial-match queries (queries that contain typos or include a subset of keywords). While centralized-index architectures (such a...

We present a censorship resistant peer-to-peer network for accessing n data items in a network of n nodes. Each search for a data item in the network takes O(log n) time and requires at most O(log2 n) messages. Our network is censorship resistant in the sense that even after adversarial removal of an arbitrarily large constant fraction of the nodes...

We consider an online version of the conflict-free coloring of a set of points on the line, where each newly inserted point must be assigned a color upon insertion, and at all times the coloring has to be conflict-free, in the sense that in every interval I there is a color that appears exactly once in I. We present deterministic and randomized alg...

We seek to understand behavior of selfish agents accessing a broadcast channel. In partic- ular, we consider the natural agent utility where costs are proportional to delay. Access to the channel is modelled as a game in extensive form with simultaneous play. Standard protocols such as Aloha are vulnerable to manipulation by selfish agents. We show...

We consider the problem of approximating a set P of n points in Rd by a collection of j-dimensional ∞ats, and ex- tensions thereof, under the standard median / mean / cen- ter measures, in which we wish to minimize, respectively, the sum of the distances from each point of P to its near- est ∞at, the sum of the squares of these distances, or the ma...

We develop efficient (1 + ")-approximation algo-rithms for generalized facility location problems. Such facilities are not restricted to being points in R,, and can represent more complex structures such as linear facili-ties (lines in R,, j-dimensional flats), etc. We introduce coresets for weighted (point) facilities. These prove to be useful for...

We present an improved online algorithm for coloring interval graphs with bandwidth. This problem has recently been studied by U. Adamy and T. Erlebach [“Online coloring of intervals with bandwidth”, Lect. Notes Comput. Sci. 2909, 1–12 (2004)] and a 195-competitive online strategy has been presented. We improve this by presenting a 10-competitive s...

We consider the following general correlation-clustering problem [N. Bansal, A. Blum, S. Chawla, Correlation clustering, in: Proc. 43rd Annu. IEEE Symp. on Foundations of Computer Science, Vancouver, Canada, November 2002, pp. 238–250]: given a graph with real nonnegative edge weights and a 〈+〉/〈-〉 edge labelling, partition the vertices into cluste...

In this paper we address the open problem of bounding the price of stability for network design with fair cost allocation for undirected graphs posed in (1). For the version of this problem that we consider, every vertex is associated with a selflsh agent, and there is a distinguished source node to which all agents must connect. We show that the p...

The common assumption about digital signatures is that they disallow any kind of modification on signed data. However, a more flexible approach is often needed and has been advocated lately, one in which some restricted modifications may still occur, without invalidating the data. This is made possible by offering signatures which are homomorphic w...

The competitive analysis fails to model locality of reference in the online paging problem. To deal with it, Borodin et. al. introduced the access graph model, which attempts to capture the locality of reference. However, the access graph model has a number of troubling aspects. The access graph has to be known in advance to the paging algorithm an...

We consider the problem of super-resolution reconstruction (SRR) in MRI. Subpixel-shifted MR images were taken in several fields of view (FOVs) to reconstruct a high-resolution image. A novel algorithm is presented. The algorithm can be applied locally and guarantees perfect reconstruction in the absence of noise. Results that demonstrate resolutio...

We deal with the problem of making capital investments in machines for manufacturing a product. Opportunities for investment occur over time, every such option consists of a capital cost for a new machine and a resulting. productivity gain, i.e., a lower production cost for one unit of product. The goal is that of minimizing the total production an...

In the present work we study the on-line call admission problem in optical networks. We present a general technique to deal with the problem of call admission and wavelength selection by reducing this problem to the problem of only call admission. We then give randomized algorithms with logarithmic competitive ratios for specific topologies in two...

Chord is a distributed hash table (DHT) that requires only O(log n) links per node and performs searches with latency and message cost O(log n), where n is the number of peers in the network. Chord assumes all nodes behave according to protocol. We give a variant of Chord which is robust with high probability for any time period during which: 1) th...

We study the role of randomization in seller optimal (i.e., profit maximization) auctions. Bayesian optimal auctions (e.g., Myerson, 1981) assume that the valuations of the agents are random draws from a distribution and prior-free optimal auctions either are randomized (e.g., Goldberg et al., 2006) or assume the valuations are randomized (e.g., Se...

We consider an online version of the conflict-free coloring of a set of points on the line, where each newly inserted point must be assigned a color upon insertion, and at all times the coloring has to be conflict-free, in the sense that in every interval I there is a color that appears exactly once in I. We present several deterministic and random...

A recent seminal result of Räcke is that for any undirected network there is an oblivious routing algorithm with a polylogarithmic competitive ratio with respect to congestion. Unfortunately, Räcke's construction is not polynomial time. We give a polynomial time construction that guarantees Räcke's bounds, and more generally gives the true optimal...

We study impurity-based decision tree algorithms such as CART, C4.5, etc., so as to better understand their theoretical underpinnings. We consider such algorithms on special forms of functions and distributions. We deal with the uniform distribution and functions that can be described as a boolean linear threshold functions or a read-once DNF. We s...

Unfair metrical task systems are a generalization of online metrical task systems. In this paper we introduce new techniques to combine algorithms for unfair metrical task systems and apply these techniques to obtain the following results: 1. Better randomized algorithms for unfair metrical task systems on the uniform metric space. 2. Better random...

We study impurity-based decision tree algorithms such as CART, C4.5, etc., so as to better understand their theoretical under- pinnings. We consider such algorithms on special forms of functions and distributions. We deal with the uniform distribution and functions that can be described as unate functions, linear threshold functions and read- once...

We consider the problem faced by a mobile robot that has to reach a given target by traveling through an unmapped region in the plane containing oriented rectangular obstacles. We assume the robot has no prior knowledge about the positions or sizes of the obstacles, and acquires such knowledge only when obstacles are encountered. Our goal is to min...

Chaum [1, 2] suggested a simple and efficient protocol aimed at providing anonymity in the presence of an adversary watching all communication links. Chaum’s protocol is known to be insecure. We show that Chaum’s protocol becomes secure when the attack model is relaxed and the adversary can control at most 99% of communication links. Our proof tech...

We present a new algorithm for mining frequent itemsets. Past studies have proposed various algorithms and techniques for improving the e#ciency of the mining task. We integrate a combination of these techniques into an algorithm which utilize those techniques dynamically according to the input dataset. The algorithm main features include depth fir...

We solve several open problems concerning the correlation clustering problem introduced by Bansal, Blum and Chawla [1]. We give an equivalence argument between these problems and the multicut problem. This implies an O(log n) approximation algorithm for minimizing disagreements on weighted and unweighted graphs. The equivalence also implies that th...

Introduction We start by discussing the PageRank algorithm, the foundation of the Google search engine. Google is a next generation search engine, that became very popular on the Internet since its creation a few years ago (a fact that empirically proves its qualities). Google is based on an algorithm called PageRank [1]. PageRank relies on the uni...

This paper deals with the file allocation problem (BFR92) con- cerning the dynamic optimization of communication costs to ac- cess data in a distributed environment. We develop a dynamic file re-allocation strategy that adapts online to a sequence of r ead and write requests whose location and relative frequencies are com- pletely unpredictable. Th...

We address a longstanding open problem of [10, 9], and present a general transformation that transforms any pointer based data structure to be confluently persistent. Such transformations for fully persistent data structures are given in [10] , greatly improving the performance compared to the naive scheme of simply copying the inputs. Unlike fully...

We introduce a problem called sequential trial optimization, a generalization of the well studied set cover problem with a new objective function. We give a simple algorithm that achieves a constant factor approximation to this problem. Sequential trial optimization naturally arises in heterogenous search environments such as peer to peer networks.

In this survey talk we discuss several problems related to peer to peer networks. A host of issues arises in the context of peer to peer networks, including efficiency issues, censorship issues, anonymity issues, etc. While many of these problems have been studied in the past, the file swapping application has taken over the Internet, given these p...

We introduce a problem called sequential trial optimization, a generalization of the well studied set cover problem with a new objective function. We give a simple algorithm that achieves a constant factor approximation to this problem. Sequential trial optimization naturally arises in heterogeneous search environments such as peer to peer networks...

The success of a P2P file-sharing network highly depends on the scalability and versatility of its search mechanism. Two particularly desirable search features are scope (ability to find infrequent items) and support for partial-match queries (queries that contain typos or include a subset of keywords). While centralized-index architectures (such a...

The Silences of the Archives, the Reknown of the Story. The Martin Guerre affair has been told many times since Jean de Coras and Guillaume Lesueur published their stories in 1561. It is in many ways a perfect intrigue with uncanny resemblance, persuasive deception and a surprizing end when the two Martin stood face to face, memory to memory, befor...

We describe mechanisms for auctions that are simultaneously truthful (alternately known as strategy-proof or incentive compatible) and guarantee high "net" profit. We make use of appropriate variants of competitive analysis of algorithms in designing and analyzing our mechanisms. Thus, we do not require any probabilistic assumptions on bids.We pres...

r complete control of a constant fraction of the nodes in the network and yet will still be unable to generate spam. Work done while on Sabbatical at University of Washington. Key Words: peer-to-peer; content addressable network (CAN); distributed hash table (DHT); censorship; spam; fault-tolerant; butterfly network; routing; Byzantine; probabilist...