An f-chromatic spanning forest of edge-colored complete bipartite graphs
  In 2001, Brualdi and Hollingsworth proved that an edge-colored balanced complete bipartite graph Kn,n with a color set C = {1,2,3,..., 2n-1} has a heterochromatic spanning tree if the number of edges colored with colors in R is more than |R|^2 /4 for any non-empty subset R \subseteq C, where a heterochromatic spanning tree is a spanning tree whose edges have distinct colors, namely, any color appears at most once. In 2010, Suzuki generalized heterochromatic graphs to f-chromatic graphs, where any color c appears at most f(c). Moreover, he presented a necessary and sufficient condition for graphs to have an f-chromatic spanning forest with exactly w components. In this paper, using this necessary and sufficient condition, we generalize the Brualdi-Hollingsworth theorem above. 
Kazuhiro Suzuki
math.CO cs.DM