arXiv subjects

Marios Mavronicolas

Publications and source records attributed to Marios Mavronicolas.

7 recordsLinked to original sources

arXiv2026

Tractable Exclusion Zones for Instant-Runoff Voting on Trees and Beyond

Instant-runoff voting (IRV) is often used when voters rank candidates rather than choosing only one favourite. We study IRV under graph-induced metric preferences where each vertex of an unweighted undirected graph hosts one voter and is also a possible candidate location. Voters rank candidates by shortest-path distance with fixed deterministic tie-breaking. We focus on exclusion zones, i.e., sets S such that, whenever at least one candidate lies in S, the IRV winner must also lie in S. Such zones serve as robustness certificates, identifying regions whose participation prevents outside winners from emerging. For general graphs, exclusion-zone verification is co-NP-complete and minimum-zone computation is NP-hard. We show that both problems become polynomial-time solvable on trees. Our main tool is a membership test asking whether a candidate can be forced to lose using opponents from a restricted region. A round-1 reduction shows that any such loss has a witness in which the candidate is eliminated in the first IRV round, enabling a bottom-up dynamic program on trees. We also show that minimum-zone computation has a much smaller search space than its definition suggests. The pairwise-loss graph, obtained from all two-candidate elections, imposes closure constraints on every exclusion zone. With deterministic tie-breaking this graph is a tournament, implying that every nonempty exclusion zone on a tree is generated by the closure of one vertex. Thus, the minimum exclusion zone can be found by testing only linearly many candidate sets. On the opposite front, we refine the intractability range of computing minimum exclusion zones on general graphs, extending it to a much broader class of deterministic elimination rules, dubbed as Strong Forced Elimination.

cs.GT↗

arXiv2023

The Contest Game for Crowdsourcing Reviews

We consider a contest game modelling a contest where reviews for $m$ proposals are crowdsourced from $n$ strategic agents} players. Player $i$ has a skill $s_{i\ell}$ for reviewing proposal $\ell$; for her review, she strategically chooses a quality $q \in \{ 1, 2, \ldots, Q \}$ and pays an effort ${\sf f}_{q} \geq 0$, strictly increasing with $q$. For her effort, she is given a strictly positive payment determined by a payment function, which is either player-invariant, like, e.g., the popular proportional allocation function, or player-specific; for a given proposal, payments are proportional to the corresponding efforts and the total payment provided by the contest organizer is 1. The cost incurred to player $i$ for each of her reviews is the difference of a skill-effort function $Λ(s_{i},{ \sf f}_{q})$ minus her payment. Skills may vary for arbitrary players and arbitrary proposals. A proposal-indifferent player $i$ has identical skills: $s_{i\ell} = s_{i}$ for all $\ell$; anonymous players means $s_{i} = 1$ for all players $i$. In a pure Nash equilibrium, no player could unilaterally reduce her cost by switching to a different quality. We present algorithmic results for computing pure Nash equilibria.

cs.GT↗

arXiv2020

(In)Existence of Equilibria for 2-Players, 2-Values Games with Concave Valuations

We consider 2-players, 2-values minimization games where the players' costs take on two values, $a,b$, $a b$, then there exists a normal 2-players, 2-values, 3-strategies game without $\mathsf{F}$-equilibrium. To the best of our knowledge, this work is the first to provide an (almost complete) answer on whether there is, for a given concave function $\mathsf{F}$, a counterexample game without $\mathsf{F}$-equilibrium.

cs.GT↗

arXiv2019

The Complexity of Computational Problems about Nash Equilibria in Symmetric Win-Lose Games

We revisit the complexity of deciding, given a {\it bimatrix game,} whether it has a {\it Nash equilibrium} with certain natural properties; such decision problems were early known to be ${\mathcal{NP}}$-hard~\cite{GZ89}. We show that ${\mathcal{NP}}$-hardness still holds under two significant restrictions in simultaneity: the game is {\it win-lose} (that is, all {\it utilities} are $0$ or $1$) and {\it symmetric}. To address the former restriction, we design win-lose {\it gadgets} and a win-lose reduction; to accomodate the latter restriction, we employ and analyze the classical {\it ${\mathsf{GHR}}$-symmetrization}~\cite{GHR63} in the win-lose setting. Thus, {\it symmetric win-lose bimatrix games} are as complex as general bimatrix games with respect to such decision problems. As a byproduct of our techniques, we derive hardness results for search, counting and parity problems about Nash equilibria in symmetric win-lose bimatrix games.

cs.CC↗

arXiv2015

The Complexity of Equilibria for Risk-Modeling Valuations

We study the complexity of deciding the existence of mixed equilibria for minimization games where players use valuations other than expectation to evaluate their costs. We consider risk-averse players seeking to minimize the sum ${\mathsf{V}} = {\mathsf{E}} + {\mathsf{R}}$ of expectation ${\mathsf{E}}$ and a risk valuation ${\mathsf{R}}$ of their costs; ${\mathsf{R}}$ is non-negative and vanishes exactly when the cost incurred to a player is constant over all choices of strategies by the other players. In a ${\mathsf{V}}$-equilibrium, no player could unilaterally reduce her cost. Say that ${\mathsf{V}}$ has the Weak-Equilibrium-for-Expectation property if all strategies supported in a player's best-response mixed strategy incur the same conditional expectation of her cost. We introduce ${\mathsf{E}}$-strict concavity and observe that every ${\mathsf{E}}$-strictly concave valuation has the Weak-Equilibrium-for-Expectation property. We focus on a broad class of valuations shown to have the Weak-Equilibrium-for-Expectation property, which we exploit to prove two main complexity results, the first of their kind, for the two simplest cases of the problem: games with two strategies, or games with two players. For each case, we show that deciding the existence of a ${\mathsf{V}}$-equilibrium is strongly ${\mathcal{NP}}$-hard for certain choices of significant valuations (including variance and standard deviation).

cs.GT↗

arXiv2012

A Distributed Algorithm for Gathering Many Fat Mobile Robots in the Plane

In this work we consider the problem of gathering autonomous robots in the plane. In particular, we consider non-transparent unit-disc robots (i.e., fat) in an asynchronous setting. Vision is the only mean of coordination. Using a state-machine representation we formulate the gathering problem and develop a distributed algorithm that solves the problem for any number of robots. The main idea behind our algorithm is for the robots to reach a configuration in which all the following hold: (a) The robots' centers form a convex hull in which all robots are on the convex, (b) Each robot can see all other robots, and (c) The configuration is connected, that is, every robot touches another robot and all robots together form a connected formation. We show that starting from any initial configuration, the robots, making only local decisions and coordinate by vision, eventually reach such a configuration and terminate, yielding a solution to the gathering problem.

cs.DC↗

arXiv2008

How Many Attackers Can Selfish Defenders Catch?

In a distributed system with {\it attacks} and {\it defenses,} both {\it attackers} and {\it defenders} are self-interested entities. We assume a {\it reward-sharing} scheme among {\it interdependent} defenders; each defender wishes to (locally) maximize her own total {\it fair share} to the attackers extinguished due to her involvement (and possibly due to those of others). What is the {\em maximum} amount of protection achievable by a number of such defenders against a number of attackers while the system is in a {\it Nash equilibrium}? As a measure of system protection, we adopt the {\it Defense-Ratio} \cite{MPPS05a}, which provides the expected (inverse) proportion of attackers caught by the defenders. In a {\it Defense-Optimal} Nash equilibrium, the Defense-Ratio is optimized. We discover that the possibility of optimizing the Defense-Ratio (in a Nash equilibrium) depends in a subtle way on how the number of defenders compares to two natural graph-theoretic thresholds we identify. In this vein, we obtain, through a combinatorial analysis of Nash equilibria, a collection of trade-off results: - When the number of defenders is either sufficiently small or sufficiently large, there are cases where the Defense-Ratio can be optimized. The optimization problem is computationally tractable for a large number of defenders; the problem becomes ${\cal NP}$-complete for a small number of defenders and the intractability is inherited from a previously unconsidered combinatorial problem in {\em Fractional Graph Theory}. - Perhaps paradoxically, there is a middle range of values for the number of defenders where optimizing the Defense-Ratio is never possible.

cs.GT↗