Do triangle-free planar graphs have exponentially many 3-colorings?

Abstract

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 $A^{\prime }$ of $A$ of size at least $\alpha |A|$ such that $G-(A\setminus A^{\prime })$ is $3$-colorable. This equivalence allows us to study restricted situations, where we can prove the statement to be true.