Counting matrix points on certain varieties over finite fields

Abstract: Classical hypergeometric functions are well-known to play an important role in arithmetic algebraic geometry. These functions offer solutions to ordinary differential equations, and special cases of such solutions are periods of Picard-Fuchs varieties of Calabi-Yau type. Gauss' $_2F_1$ includes the celebrated case of elliptic curves through the theory of elliptic functions. In the 80s, Greene defined finite field hypergeometric functions that can be used to enumerate the number of finite field points on such varieties. We extend some of these results to count finite field ``matrix points." For example, for every $n\geq 1,$ we consider the matrix elliptic curves $$ B^2 = A(A-I_n)(A-a I_n), $$ where $(A,B)$ are commuting $n\times n$ matrices over a finite field $\mathbb{F}_q$ and $a\neq 0,1$ is fixed. Our formulas are assembled from Greene's hypergeometric functions and $q$-multinomial coefficients. We use these formulas to prove Sato-Tate distributions for the error terms for matrix point counts for these curves and some families of $K3$ surfaces.