Automatic congruences for diagonals of rational functions

Abstract: In this paper we use the framework of automatic sequences to study combinatorial sequences modulo prime powers. Given a sequence whose generating function is the diagonal of a rational power series, we provide a method, based on work of Denef and Lipshitz, for computing a finite automaton for the sequence modulo $p^\alpha$, for all but finitely many primes $p$. This method gives completely automatic proofs of known results, establishes a number of new theorems for well-known sequences, and allows us to resolve some conjectures regarding the Ap\'ery numbers. We also give a second method, which applies to an algebraic sequence modulo $p^\alpha$ for all primes $p$, but is significantly slower. Finally, we show that a broad range of multidimensional sequences possess Lucas products modulo $p$.