Assertion: dual graph | /r/HasProperty | mentioned by alfred kempe
Facets:
value: in 1879
type: TEMPORAL
Source sentences:
text: In connection with the four color theorem, the dual graphs of maps (subdivisions of the plane into regions) were mentioned by Alfred Kempe in 1879, and extended to maps on non-planar surfaces by Lothar Heffter [de] in 1891.