A new proof of Grünbaum's 3 color theorem

O.V. Borodin
1997 Discrete Mathematics  
A simple proof of Grfinbaum's theorem on the 3-colourability of planar graphs having at most three 3-cycles is given, which does not employ the colouring extension. In 1958, Gr6tzsch I-5] proved that every planar graph without cycles of length three is 3-colourable. In 1963, Griinbaum [6] extended this result as follows: Theorem 1. Every planar graph with at most three 3-cycles is 3-colourable.
doi:10.1016/0012-365x(95)00984-5 fatcat:qre273wsorbexkypwrqra3adna