Congruences for Franel numbers

Zhi-Wei Sun

Published 2011-12-05, updated 2013-10-29Version 11

The Franel numbers given by $f_n=\sum_{k=0}^n\binom{n}{k}^3$ ($n=0,1,2,\ldots$) play important roles in both combinatorics and number theory. In this paper we initiate the systematic investigation of fundamental congruences for the Franel numbers. We mainly establish for any prime $p>3$ the following congruences: \begin{align*}\sum_{k=0}^{p-1}(-1)^kf_k&\equiv\left(\frac p3\right)\ \ (\mbox{mod}\ p^2), \\ \sum_{k=0}^{p-1}(-1)^k\,kf_k&\equiv-\frac 23\left(\frac p3\right)\ \ (\mbox{mod}\ p^2), \\ \sum_{k=1}^{p-1}\frac{(-1)^k}kf_k &\equiv0\ \ (\mbox{mod}\ p^2), \\ \sum_{k=1}^{p-1}\frac{(-1)^k}{k^2}f_k&\equiv0\ \ (\mbox{mod}\ p). \end{align*}