Congruences involving $g_n(x)=\sum_{k=0}^n\binom nk^2\binom{2k}kx^k$

Zhi-Wei Sun

Published 2014-07-03, updated 2014-12-23Version 5

Define $g_n(x)=\sum_{k=0}^n\binom nk^2\binom{2k}kx^k$ for $n=0,1,2,\ldots$. Those numbers $g_n=g_n(1)$ are closely related to Ap\'ery numbers and Franel numbers. In this paper we establish some fundamental congruences involving $g_n(x)$. For example, for any prime $p>5$ we have $$\sum_{k=1}^{p-1}\frac{g_k(-1)}{k}\equiv 0\pmod{p^2}\quad\mbox{and}\quad\sum_{k=1}^{p-1}\frac{g_k(-1)}{k^2}\equiv 0\pmod p.$$ This is similar to Wolstenholme's classical congruences $$\sum_{k=1}^{p-1}\frac1k\equiv0\pmod{p^2}\quad\mbox{and}\quad\sum_{k=1}^{p-1}\frac{1}{k^2}\equiv0\pmod p$$ for any prime $p>3$.