Do Triangle-Free Planar Graphs have Exponentially Many $3$-Colorings?

Zdeněk Dvořák, Jean-Sébastien Sereni


Thomassen conjectured that triangle-free planar graphs have an exponential number of $3$-colorings. We show this conjecture to be equivalent to the following statement: there exists a positive real $\alpha$ such that whenever $G$ is a planar graph and $A$ is a subset of its edges whose deletion makes $G$ triangle-free, there exists a subset $A'$ of $A$ of size at least $\alpha|A|$ such that $G-(A\setminus A')$ is $3$-colorable. This equivalence allows us to study restricted situations, where we can prove the statement to be true.


Many 3-colorings; Planar graphs; Triangle-free graphs; Grötzsch's theorem

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