Abel’s Problem, Gauss and Cartier Congruences Over Number Fields

Abel’s problem consists in identifying the conditions under which the differential equation y^=η y, with η an algebraic function in C(x), possesses a non-zero algebraic solution y. This problem has been algorithmically solved by Risch. In a previous paper, we presented an alternative solution in the special arithmetic situation where η has a Puiseux expansion with rational coefficients at the origin: there exists a non-trivial algebraic solution of y^=η y if and only if the coefficients of the Puiseux expansion of xη (x) at $0$ satisfy Gauss congruences for almost all prime numbers. In this paper, we generalize this criterion to arbitrary η algebraic over Q̄(x), by means of a natural generalization to number fields of Gauss congruences and of the weaker Cartier congruences recently introduced in this context by Bostan. We then provide applications of this criterion in the hypergeometric setting and for Artin–Mazur zeta functions.