Noncommutative Valiant's Classes: Structure and Complete Problems

TL;DR

This paper investigates the structure of noncommutative algebraic complexity classes, establishing completeness results for Dyck and Palindrome polynomials, and demonstrating hierarchies within these classes under various reductions.

Contribution

It introduces noncommutative analogues of Valiant's classes, proves completeness of Dyck and Palindrome polynomials, and reveals hierarchies within these classes under specific reductions.

Findings

Dyck polynomials are complete for VP_nc under abp reductions.

Palindrome polynomials are complete for VSKEW_nc under abp reductions.

There exists a strict hierarchy inside VNP_nc and VP_nc based on polynomial nesting depth.

Abstract

In this paper we explore the noncommutative analogues, $\mathrm{VP}_{nc}$ and $\mathrm{VNP}_{nc}$, of Valiant's algebraic complexity classes and show some striking connections to classical formal language theory. Our main results are the following: (1) We show that Dyck polynomials (defined from the Dyck languages of formal language theory) are complete for the class $\mathrm{VP}_{nc}$ under $\le_{abp}$ reductions. Likewise, it turns out that $\mathrm{PAL}$ (Palindrome polynomials defined from palindromes) are complete for the class $\mathrm{VSKEW}_{nc}$ (defined by polynomial-size skew circuits) under $\le_{abp}$ reductions. The proof of these results is by suitably adapting the classical Chomsky-Sch\"{u}tzenberger theorem showing that Dyck languages are the hardest CFLs. (2) Next, we consider the class $\mathrm{VNP}_{nc}$. It is known~\cite{HWY10a} that, assuming the sum-of-squares conjecture, the noncommutative polynomial $\sum_{w\in\{x_0,x_1\}^n}ww$ requires exponential size circuits. We unconditionally show that $\sum_{w\in\{x_0,x_1\}^n}ww$ is not $\mathrm{VNP}_{nc}$-complete under the projection reducibility. As a consequence, assuming the sum-of-squares conjecture, we exhibit a strictly infinite hierarchy of p-families under projections inside $\mathrm{VNP}_{nc}$ (analogous to Ladner's theorem~\cite{Ladner75}). In the final section we discuss some new $\mathrm{VNP}_{nc}$-complete problems under $\le_{abp}$-reductions. (3) Inside $\mathrm{VP}_{nc}$ too we show there is a strict hierarchy of p-families (based on the nesting depth of Dyck polynomials) under the $\le_{abp}$ reducibility.