Lothar Heffter

rotation systems and 2-cell-embeddings was first settled in a dual form by Lothar Heffter in the 1890s and extensively used by Ringel during the 1950s. Independently

and Judith (born 1943). Peltesohn solved the Difference Problems of Lothar Heffter [de] (1896) in combinatorics in 1939. A Difference Triple (a, b, c)

Alfred Kempe in 1879, and extended to maps on non-planar surfaces by Lothar Heffter [de] in 1891. Duality as an operation on abstract planar graphs was

field as a module over its abelian Galois group (cf Iwasawa theory). Lothar Heffter, Ludwig Stickelberger, Jahresbericht der Deutschen Matematische Vereinigung

1090/s0002-9904-1944-08150-0, MR 1564626, available from Project Euclid. Heffter, Lothar (1943), "Oskar Bolza", Jahresbericht der Deutschen Mathematiker-Vereinigung