The trace of Frobenius of elliptic curves and the $p$-adic gamma function

Abstract

We define a function in terms of quotients of the $p$-adic gamma function which generalizes earlier work of the author on extending hypergeometric functions over finite fields to the $p$-adic setting. We prove, for primes $p>3$, that the trace of Frobenius of any elliptic curve over ${\mathbb{F}}_p$, whose $j$-invariant does not equal 0 or 1728, is just a special value of this function. This generalizes results of Fuselier and Lennon which evaluate the trace of Frobenius in terms of hypergeometric functions over ${\mathbb{F}}_p$ when $p\equiv 1(\mathrm{mod}12)$.