Some supercongruences occurring in truncated hypergeometric series

Abstract

For the purposes of this paper supercongruences are congruences between terminating hypergeometric series and quotients of $p$-adic Gamma functions that are stronger than those one can expect to prove using commutative formal group laws. We prove a number of such supercongruences by using classical hypergeometric transformation formulae. These formulae (see the appendix), most of which are decades or centuries old, allow us to write the terminating series as the ratio of products of of $\Gamma$-values. At this point sums have become quotients. Writing these $\Gamma$-quotients as $\Gamma_p$-quotients, we are in a situation that is well-suited for proving $p$-adic congruences. These $\Gamma_p$-functions can be $p$-adically approximated by their Taylor series expansions. Sometimes there is cancelation of the lower order terms, leading to stronger congruences. Using this technique we prove, among other things, a conjecture of Kibelbek and a strengthened version of a conjecture of van Hamme.