Revised: October 2, 2018

Published: October 15, 2019

**Keywords:**complexity theory, complexity, combinatorics, additive combinatorics, algebraic complexity, circuit complexity, arithmetic circuits, lower bounds, rank, polynomials, matrix rigidity, polynomial method, Hamming distance

**Categories:**complexity theory, lower bounds, combinatorics, additive combinatorics, algebraic complexity, circuit complexity, arithmetic circuits, rank, polynomials, matrix rigidity, polynomial method, Hamming distance, note

**ACM Classification:**F.2.2, F.1.3

**AMS Classification:**68Q17, 68Q15

**Abstract:**
[Plain Text Version]

Matrix rigidity is a notion put forth by Valiant (1977) as a means for proving arithmetic circuit lower bounds. A matrix is rigid if it is far, in Hamming distance, from any low-rank matrix. Despite decades of effort, no explicit matrix rigid enough to carry out Valiant's plan has been found. Recently, Alman and Williams (STOC'17) showed that, contrary to common belief, the Walsh--Hadamard matrices cannot be used for Valiant's program as they are not sufficiently rigid.

Our main result is a similar non-rigidity theorem for *any*
$q^n \times q^n$ matrix $M$ of the form $M(x,y) = f(x+y)$, where
$f:\mathbb{F}_q^n \to \mathbb{F}_q$ is any function and $\mathbb{F}_q$
is a fixed finite field of $q$ elements ($n$ goes to infinity). The
theorem follows almost immediately from a recent lemma of Croot, Lev
and Pach (2017) which is also the main ingredient in the recent
solution of the famous cap-set problem by Ellenberg and Gijswijt
(2017).