On computing the smallest four-coloring of planar graphs and non-self-reducible sets in P.

André Große Jörg Rothe Gerd Wechsung On computing the smallest four-coloring of planar graphs and non-self-reducible sets in P. 215-221 2006 99 Inf. Process. Lett. 6 http://dx.doi.org/10.1016/j.ipl.2006.04.007 db/journals/ipl/ipl99.html#GrosseRW06