During my doctoral study days at the University of Oklahoma I knew several mathematicians who were working on graph theory problems -- and a couple of them worked on problems related to the Four Color Conjecture -- a conjecture (dating back to around 1850) that became a theorem in 1976 with a proof by Kenneth Appel and Wolfgang Haken verifying (using many hours of computer time). It asserts that four colors are sufficient to color any plane map so that no pair of adjacent regions have the same color. This theorem has been again on my mind since reading the obituary of Kenneth Appel, who died on April 19.
Here is a link to an earlier posting (5 November 2011) on the Four Color Problem with a poem by Frank Bernhart. And here, repeated from that post, is my poetic version of the Four Color Theorem: