Gerhard Ringel

Gerhard Ringel (October 28, 1919, in Kollnbrunn, Austria – June 24, 2008, in Santa Cruz, California) was a German mathematician who earned his Ph.D. from the University of Bonn in 1951. He was one of the pioneers in graph theory and contributed significantly to the proof of the Heawood conjecture (now the Ringel-Youngs theorem), a mathematical problem closely linked with the Four Color Theorem.
Although born in Kollnbrunn, Ringel was raised in Czechoslovakia and attended Charles University before being drafted into the German Army in 1940 (after Germany had taken control of much of what had been Czechoslovakia. After the war Ringel served for over four years in a Soviet prisoner of war camp.
Gerhard Ringel started his academic career as professor at the Free University Berlin. In 1970 he left Germany due to bureaucratic consequences of the German student movement, and continued his career at the University of California, Santa Cruz, having been invited there by his coauthor, Professor John W. T. (Ted) Youngs. He was awarded honorary doctorate degrees from the University of Karlsruhe (TH) and the Free University Berlin.
Besides his mathematical skills he was a widely acknowledged entomologist. His main emphasis lay on collecting and breeding butterflies. Prior to his death, he gave his outstanding collection of butterflies to the UCSC Museum of Natural History Collections.

• Ringel, Gerhard; Youngs, J.W.T. (1968). "Solution of the Heawood map-coloring problem". Proc. Natl. Acad. Sci. USA. 60 (2): 438–445. doi:10.1073/pnas.60.2.438. MR 0228378. PMC 225066. PMID 16591648. 
• Ringel, Gerhard (1974). Map Color Theorem. New York/Berlin: Springer-Verlag. 
• Hartsfield, Nora; Ringel, Gerhard (1990). Pearls in graph theory. Academic Press, Boston, MA. ISBN 0-12-328552-6. 

• Kleitman, Daniel (1975). "Review: Map color theorem by Gerhard Ringel" (PDF). Bull. Amer. Math. Soc. 81 (4): 657–660. doi:10.1090/s0002-9904-1975-13811-0.