On Tree Automata, Generating Functions, and Differential Equations

Abstract

In this paper we introduce holonomic tree automata: a common extension of weighted tree automata and holonomic recurrences. We show that the generating function of the tree series represented by such an automaton is differentially algebraic. Conversely, we give an algorithm that inputs a differentially algebraic power series, represented as a solution of a rational dynamical system, and outputs an automaton whose generating function is the given series. Such an automaton yields a recurrence that can be used to compute the terms of the power series. We use the algorithm to obtain automaton representations of exponential generating functions of families of combinatorial objects given as combinatorial species. Using techniques from differential algebra, we show that it is decidable both whether two automata represent the same formal tree series and whether they have the same generating function.