A Topological Characterization of Modulo-$p$ Arguments and Implications for Necklace Splitting

Abstract

The classes PPA-p have attracted attention lately, because they are the main candidates for capturing the complexity of Necklace Splitting with p thieves, for prime p. However, these classes are not known to have complete problems of a topological nature, which impedes any progress towards settling the complexity of the problem. On the contrary, such problems have been pivotal in obtaining completeness results for PPAD and PPA, for several important problems, such as finding a Nash equilibrium [Daskalakis et al., 2009, Chen et al., 2009b and Necklace Splitting with 2 thieves [Filos- Ratsikas and Goldberg, 2019].In this paper, we provide the first topological characterization of the classes PPA-p. First, we show that the computational problem associated with a simple generalization of Tucker's Lemma, termed p-polygon-Tucker, as well as the associated Borsuk-Ulam-type theorem, p-polygon-Borsuk-Ulam, are PPA-p-complete. Then, we show that the computational version of the well-known BSS Theorem [Bárány et al., 1981], as well as the associated BSS-Tucker problem are PPA-p-complete. Finally, using a different generalization of Tucker's Lemma (termed Z p -star-Tucker), which we prove to be PPA-p-complete, we prove that p-thieves Necklace Splitting is in PPA-p. All of our containment results are obtained through a new combinatorial proof for Z p -versions of Tucker's lemma that is a natural generalization of the standard combinatorial proof of Tucker's lemma by Freund and Todd [1981]. We believe that this new proof technique is of independent interest.

The Complexity of Constrained Min-Max Optimization

“…Beyond equilibrium computation and its applications to Economics and Game Theory, the study of total search problems has found profound connections to many scientific fields, including continuous optimization [DP11,DTZ18], combinatorial optimization [SY91], query complexity [BCE + 95], topology [GH19], topological combinatorics and social choice theory [FG18,FG19,FRHSZ20b,FRHSZ20a], algebraic combinatorics [BIQ + 17, GKSZ19], and cryptography [Jeř16,BPR15,SZZ18]. For a more extensive overview of total search problems we refer the reader to the recent survey by Daskalakis [Das18].…”