Matrix rigidity and the Croot-Lev-Pach lemma

Matrix rigidity is a notion put forth by Valiant [Val77] 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 efforts, no explicit matrix rigid enough to carry out Valiant’s plan has been found. Recently, Alman and Williams [AW17] showed, contrary to common belief, that the 2 × 2 Hadamard matrix Hn = ( (−1) ) x,y∈Fn 2 could not be used for Valiant’s program as it is not sufficiently rigid. In this note we observe a similar ‘non rigidity’ phenomena for any q × q matrix M of the form M(x, y) = f(x + y), where f : F q → Fq is any function and Fq 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 [CLP17] which is also the main ingredient in the recent solution of the cap-set problem [EG17].