Exponential growth of rank jumps for A-hypergeometric systems

The dimension of the space of holomorphic solutions at nonsingular points (also called the holonomic rank) of a $A$--hypergeometric system $M_A (β)$ is known to be bounded above by $2^{2d}\operatorname{vol}(A)$, where $d$ is the rank of the matrix $A$ and $\vol (A)$ is its normalized volume. This bound was thought to be very vast because it is exponential on $d$. Indeed, all the examples we have found in the literature verify that $\operatorname{rank}(M_A (β))<2 \vol (A)$. We construct here, in a very elementary way, some families of matrices $A_{(d)}\in \ZZ^{d \times n}$ and parameter vectors $β_{(d)} \in \C^d$, $d\geq 2$, such that $\rank (M_{A_{(d)}} (β_{(d)}))\geq a^d \vol(A_{(d)})$ for certain $a>1$.

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