Marco Carmosino ; Ronald Fagin ; Neil Immerman ; Phokion Kolaitis ; Jonathan Lenchner et al. - Multi-Structural Games and Beyond

lmcs:11364 - Logical Methods in Computer Science, December 24, 2024, Volume 20, Issue 4 - https://doi.org/10.46298/lmcs-20(4:27)2024
Multi-Structural Games and BeyondArticle

Authors: Marco Carmosino ; Ronald Fagin ; Neil Immerman ; Phokion Kolaitis ; Jonathan Lenchner ; Rik Sengupta

    Multi-structural (MS) games are combinatorial games that capture the number of quantifiers of first-order sentences. On the face of their definition, MS games differ from Ehrenfeucht-Fraisse (EF) games in two ways: first, MS games are played on two sets of structures, while EF games are played on a pair of structures; second, in MS games, Duplicator can make any number of copies of structures. In the first part of this paper, we perform a finer analysis of MS games and develop a closer comparison of MS games with EF games. In particular, we point out that the use of sets of structures is of the essence and that when MS games are played on pairs of structures, they capture Boolean combinations of first-order sentences with a fixed number of quantifiers. After this, we focus on another important difference between MS games and EF games, namely, the necessity for Spoiler to play on top of a previous move in order to win some MS games. Via an analysis of the types realized during MS games, we delineate the expressive power of the variant of MS games in which Spoiler never plays on top of a previous move. In the second part we focus on simultaneously capturing number of quantifiers and number of variables in first-order logic. We show that natural variants of the MS game do *not* achieve this. We then introduce a new game, the quantifier-variable tree game, and show that it simultaneously captures the number of quantifiers and number of variables. We conclude by generalizing this game to a family of games, the *syntactic games*, that simultaneously capture reasonable syntactic measures and the number of variables.


    Volume: Volume 20, Issue 4
    Published on: December 24, 2024
    Accepted on: November 22, 2024
    Submitted on: May 24, 2023
    Keywords: Computer Science - Logic in Computer Science,Computer Science - Computational Complexity