Volume 11 (2015) Article 14 pp. 357-393

Non-Commutative Arithmetic Circuits with Division

by Pavel Hrubeš and Avi Wigderson

Received: January 19, 2013
Revised: September 19, 2015
Published: December 20, 2015

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We initiate the study of the complexity of arithmetic circuits with division gates over non-commuting variables. Such circuits and formulae compute non-commutative rational functions, which, despite their name, can no longer be expressed as ratios of polynomials. We prove some lower and upper bounds, completeness and simulation results, as follows.

If $X$ is an $n\times n$ matrix consisting of $n^2$ distinct mutually non-commuting variables, we show that:

1. $X^{-1}$ can be computed by a circuit of polynomial size.
2. Every formula computing some entry of $X^{-1}$ must have size at least $2^{\Omega(n)}$.

We also show that matrix inverse is complete in the following sense:

1. Assume that a non-commutative rational function $f$ can be computed by a formula of size $s$. Then there exists an invertible $2s\times 2s$-matrix $A$ whose entries are variables or field elements such that $f$ is an entry of $A^{-1}$.
2. If $f$ is a non-commutative polynomial computed by a formula without inverse gates then $A$ can be taken as an upper triangular matrix with field elements on the diagonal.

We show how divisions can be eliminated from non-commuta\-tive circuits and formulae which compute polynomials, and we address the non-commutative version of the “rational function identity testing” problem. As it happens, the complexity of both of these procedures depends on a single open problem in invariant theory.

DOI: 10.4086/toc.2015.v011a014

© 2015 Pavel Hrubeš and Avi Wigderson
Licensed under a Creative Commons Attribution License (CC-BY)