Michal Malafiejski Krzysztof Giaro Robert Janczewski Marek Kubale Sum Coloring of Bipartite Graphs with Bounded Degree. 235-244 2004 40 Algorithmica 4 http://dx.doi.org/10.1007/s00453-004-1111-4 db/journals/algorithmica/algorithmica40.html#MalafiejskiGJK04