Algebraic independence and linear difference equations

TL;DR

This paper proves algebraic independence of solutions to different types of linear difference equations under certain conditions, confirming a longstanding conjecture and applying Galois theory to analyze transcendence.

Contribution

It establishes algebraic independence results for solutions of linear $ au$-equations with different automorphisms, settling a conjecture about Mahler functions and introducing a Galois-theoretic approach.

Findings

Proves algebraic independence of solutions to shift, q-difference, and Mahler equations.

Confirms a 1987 conjecture on Mahler functions.

Provides a general Galois-theoretic framework for difference equations.

Abstract

We consider pairs of automorphisms $(\phi,\sigma)$ acting on fields of Laurent or Puiseux series: pairs of shift operators $(\phi\colon x\mapsto x+h_1, \sigma\colon x\mapsto x+h_2)$, of $q$-difference operators $(\phi\colon x\mapsto q_1x,\ \sigma\colon x\mapsto q_2x)$, and of Mahler operators $(\phi\colon x\mapsto x^{p_1},\ \sigma\colon x\mapsto x^{p_2})$. Given a solution $f$ to a linear $\phi$-equation and a solution $g$ to a linear $\sigma$-equation, both transcendental, we show that $f$ and $g$ are algebraically independent over the field of rational functions, assuming that the corresponding parameters are sufficiently independent. As a consequence, we settle a conjecture about Mahler functions put forward by Loxton and van der Poorten in 1987. We also give an application to the algebraic independence of $q$-hypergeometric functions. Our approach provides a general strategy to study this kind of question and is based on a suitable Galois theory: the $\sigma$-Galois theory of linear $\phi$-equations.