German mathematician and educator whose study of topology in 1910 led to his theorem on topological manifolds, known as Dehn's lemma.

Dehn was educated in Germany and received his doctorate from the University of Göttingen in 1900. He was influenced by the German mathematician David Hilbert (Hilbert, David)'s work on axiomatization of geometry as well as by the writings of the French mathematician, Henri Poincaré (Poincaré, Henri). Dehn served as professor of mathematics at Frankfurt University from 1921 until 1935, when he was forced to leave the university by the Nazis. In 1940 Dehn immigrated to the United States to continue his teaching and topological research.