nontrivial

nontrivial adj.  Requiring real thought or significant
   computing power.  Often used as an understated way of saying that a
   problem is quite difficult or impractical, or even entirely
   unsolvable ("Proving P=NP is nontrivial").  The preferred
   emphatic form is `decidedly nontrivial'.  See {trivial},
   {uninteresting}, {interesting}.