Some acyclic systems of permutations are not realizable by   triangulations of a product of simplices
  The acyclic system conjecture of Ardila and Ceballos can be interpreted as saying the following: "Every triangulation of the 3-skeleton of a product of two simplices can be extended to a triangulation of the whole product". We show a counter-example to this.   Motivation for this conjecture comes from a related conjecture, the "spread-out simplices" conjecture of Ardila and Billey. We give some necessary conditions that counter-examples to this second conjecture (if they exist) must satisfy. 
Francisco Santos
math.CO