Integrality of Power Expansions Related to Hypergeometric Series

W. V. Zudilin

Matematičeskie zametki, Tome 71 (2002) no. 5, pp. 662-676

Résumé

In the present paper, we study the arithmetic properties of power expansions related to generalized hypergeometric differential equations and series. Defining the series $f(z),g(z)$ in powers of $z$ so that $f(z)$ and $f(z)\log z+g(z)$ satisfy a hypergeometric equation under a special choice of parameters, we prove that the series $q(z)=ze^{g(Cz)/f(Cz)}$ in powers of $z$ and its inversion $z(q)$ in powers of $q$ have integer coefficients (here the constant $C$ depends on the parameters of the hypergeometric equation). The existence of an integral expansion $z(q)$ for differential equations of second and third order is a classical result; for orders higher than 3 some partial results were recently established by Lian and Yau. In our proof we generalize the scheme of their arguments by using Dwork's $p$-adic technique.