arXiv subjects

Moshe Y. Vardi

Publications and source records attributed to Moshe Y. Vardi.

3 recordsLinked to original sources

arXiv2026

From Ramsey-Based to Congruence-Based Constructions for Büchi Complementation

The very first construction by J. Richard Büchi himself for complementing a Büchi automaton relies on a fundamental lemma about the division of an arbitrary infinite word into consecutive finite words, which was cleanly proven by invoking a specialized theorem of Ramsey. For that reason, constructions of similar nature have subsequently been labeled as Ramsey-based. Nevertheless, it suffices to have a weaker form of the lemma where the finite words come from a finite number of congruence classes, rather than arbitrary classes, that form a partition of the set of all finite words. The weaker lemma, with support of nicer properties from a congruence, can be proven without Ramsey's theorem. A commonly adopted improvement on such complementation constructions also requires the working of a congruence. This paper recounts the history and reviews using more contemporary terminology wherever possible the relevant concepts and results, to advocate renaming of Ramsey-based constructions as congruence-based constructions.

cs.FL↗

arXiv2026

Target Discounted Sum Problem on Markov Chains with Applications to Markov Decision Processes

The discounted sum is a way to aggregate a sequence of weights from a finite alphabet $Σ$, i.e., for a discount factor $λ$, the discounted sum of a sequence $w_0 w_1 w_2 \cdots$ over $Σ$ is $\sum_{i \in \mathbb{N}} w_i λ^i$. The target discounted-sum problem, which is currently open, asks, given $λ,Σ$ and a target $t$, whether there exists an infinite sequence over $Σ$ whose discounted sum is equal to $t$. We study and solve a probabilistic variant of this problem, i.e., the target discounted-sum problem on Markov chains. To do this, we prove that the event consisting of paths whose discounted sum is equal to the target and has infinitely many distinct suffix sums has probability zero. This structural property allows us to solve the target discounted-sum problem on Markov chains using an automata-theoretic technique. We apply our technical results to Markov decision processes with target discounted-sum objectives: we show that the infimum value and the finite-memory supremum value are computable in pseudo-polynomial time and are attained by deterministic finite-memory strategies.

cs.LO↗

arXiv2026

A Numerical Approach to the Realizability Problems for Memoryless Nash and Epsilon Equilibria in Concurrent Multiplayer Reachability Games

The existence of equilibria, which is called the realizability problem in the formal methods community, for probabilistic undiscounted state-based systems has proven to be an incredibly challenging research setting. In this paper, we consider a restricted version of the classic realizability problem by focusing on equilibria that are both memoryless and numerically constrained. While the restriction to memoryless strategies is relatively common in the literature, numerical constraints, to the best of our knowledge, represent a new and powerful approach. First, we consider the existence of memoryless equilibria when all numbers involved must be rational numbers of a pre-defined limited size. Then, we extend this analysis to field extensions of the rational numbers created through radical algebraic generators. When the basis is provided for the latter, both realizability problems are NP-complete for both exact and epsilon Nash equilibria. Finally, we consider an unconstrained numerical setting. While the characterization of the exact Nash equilibria realizability problem as ETR-complete is one of the most celebrated results in the literature, we demonstrate that the constructions underlying this result are flawed as presented. We then mend said constructions to preserve the ETR upper bound, and note that the lower bound in the literature does not apply to the epsilon-equilibrium setting. Overall, this paper demonstrates an interesting relationship between the complexity of the realizability problem and the "complexity" of the numbers involved in the representations of strategies.

cs.GT↗