Planar 4-critical graphs with four triangles

Oleg V. Borodin, Zdeněk Dvořák, Alexandr V. Kostochka +2

By the Grunbaum-Aksenov Theorem (extending Grotzsch's Theorem) every planar graph with at most three triangles is 3-colorable. However, there are infinitely many planar 4-critical…