On heterochromatic out-directed spanning trees in tournaments
  Given a tournament T, let h(T) be the smallest integer k such that every arc-coloring of T with k or more colors produces at least one out-directed spanning tree of T with no pair of arcs with the same color. In this paper we give the exact value of h(T). 
Juan Jos\'e Montellano-Ballesteros, Eduardo Rivera Campo
math.CO