The Emptiness Problem for Tree Automata with at Least One Disequality   Constraint is NP-hard
  The model of tree automata with equality and disequality constraints was introduced in 2007 by Filiot, Talbot and Tison. In this paper we show that if there is at least one disequality constraint, the emptiness problem is NP-hard. 
P.-C H\'eam (INRIA Nancy - Grand Est / LORIA / LIFC, FEMTO-ST), V.   Hugot (INRIA Lille - Nord Europe, LIFL), O. Kouchnarenk...
cs.FL