Let $A$ be a set of $N$ vectors in ${ \mathbb Z}^n$ and let $v$ be a vector in ${ \mathbb C}^N$ that has minimal negative support for $A$. Such a vector $v$ gives rise to a formal series solution of the $A$-hypergeometric system with parameter $\beta = Av$. If $v$ lies in ${ \mathbb Q}^n$, then this series has rational coefficients. Let $p$ be a prime number. We characterize those $v$ whose coordinates are rational, $p$-integral, and lie in the closed interval $[-1,0]$ for which the corresponding normalized series solution has $p$-integral coefficients.
Categories: math.NT math.AG