Grünbaum's torus conjecture for cubic graph colorings

Let GG be a 22-connected cubic graph embedded in the torus. The graph GG is 3-edge-colorable if its edges can be colored with three colors so that adjacent edges receive different colors. Grünbaum's torus conjecture. If GG is not 3-edge-colorable, then GG contains two edges whose removal gives a planar graph. Grünbaum's broader conjecture for cubic graphs with polyhedral embeddings in orientable surfaces was disproved in genus 55 by Kochol. The torus case remains unresolved; the conjecture is motivated by examples of toroidal snarks obtained from Petersen-graph constructions.

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Primary source

Yuta Inoue, Ken-ichi Kawarabayashi, Atsuyuki Miyashita, Bojan Mohar and Tomohiro Sonobe, “Three-edge-coloring projective planar cubic graphs: A generalization of the Four Color Theorem”, arXiv:2405.16586 (2024).

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