Popular differences for matrix patterns

Abstract: The following combinatorial conjecture arises naturally from recent ergodic-theoretic work of Ackelsberg, Bergelson, and Best. Let $M_1$, $M_2$ be $k\times k$ integer matrices, $G$ be a finite abelian group of order $N$, and $A\subseteq G<sup>k$ with $|A|\ge\alpha N<sup>k$. If $M_1$, $M_2$, $M_1-M_2$, and $M_1+M_2$ are automorphisms of $G<sup>k$, is it true that there exists a popular difference $d \in G<sup>k\setminus{0}$ such that [#{x \in G^k: x, x+M_1d, x+M_2d, x+(M_1+M_2)d \in A} \ge (\alpha^4-o(1))N^k.] We show that this conjecture is false in general, but holds for $G = \mathbb{F}_p<sup>n$ with $p$ an odd prime given the additional spectral condition that no pair of eigenvalues of $M_1M_2<sup>{-1}$ (over $\overline{\mathbb{F}}_p$) are negatives of each other. In particular, the "rotated squares" pattern does not satisfy this eigenvalue condition, and we give a construction of a set of positive density in $(\mathbb{F}_5<sup>n)<sup>2$ for which that pattern has no nonzero popular difference. This is in surprising contrast to three-point patterns, which we handle over all compact abelian groups and which do not require an additional spectral condition.