A note on Christol's theorem

Abstract

Christol's theorem characterises algebraic power series over finite fields in terms of finite automata. In a recent article, Bridy develops a new proof of Christol's theorem by Speyer, to obtain a tight quantitative version, that is, to bound the size of the corresponding automaton in terms of the height and degree of the power series, as well as the genus of the curve associated with the minimal polynomial of the power series. Speyer's proof, and Bridy's development, both take place in the setting of algebraic geometry, in particular by considering K\"ahler differentials of the function field of the curve. In this note we show how an elementary approach, based on diagonals of bivariate rational functions, provides essentially the same bounds.