Duality and monodromy reducibility of $A$-hypergeometric systems

We study $A$-hypergeometric systems $H_A(\beta)$ in the sense of Gelfand, Kapranov and Zelevinsky under two aspects: the structure of their holonomically dual system, and reducibility of their rank module. We prove first that rank-jumping parameters always correspond to reducible systems, and we show that the property of being reducible is ``invariant modulo the lattice''. In the second part we study a conjecture of Nobuki Takayama which states that the holonomic dual of $H_A(\beta)$ is of the form $H_A(\beta')$ for suitable $\beta'$. We prove the conjecture for all matrices $A$ and generic parameter $\beta$, exhibit an example that shows that in general the conjecture cannot hold, and present a refined version of the conjecture. Questions on both duality and reducibility have been impossible to answer with classical methods. This paper may be seen as an example of the usefulness, and scope of applications, of the homological tools for $A$-hypergeometric systems developed in \cite{MMW}.