Rank jumps in Codimension 2 A-Hypergeometric Systems
  The holonomic rank of the A-hypergeometric system H_A(\beta) is shown to depend on the parameter vector \beta when the underlying toric ideal I_A is a non Cohen Macaulay codimension 2 toric ideal. The set of exceptional parameters is usually infinite. 
Laura Felicia Matusevich (UC Berkeley)