Two counterexamples for power ideals of arrangements
  We disprove Holtz and Ron's conjecture that the power ideal C_{A,-2} of a hyperplane arrangement A (also called the "internal zonotopal space") is generated by A-monomials. We also show that, in contrast with the case k \geq -2, the Hilbert series of C_{A,k} is not determined by the matroid of A for k \leq -6. 
Federico Ardila, Alex Postnikov
math.CO math.AC