# Is 4-Coloring restricted to graphs with crossing number 1 NP-complete?

Planar graphs are 4-colorable. Determining if a planar graph is 3-colorable is NP-Complete.

A graph with a crossing number 1 (graph such that it can be drawn with $$\le 1$$ crossing) is 5-colorable.

What is the hardness of determining if a graph with crossing number 1 is 4-colorable? Is that NP-Complete?

One can also can ask similar question for crossing numbers 2,3,... and for genus 1,2,...

• It seems equivalent to checking if two nodes are "fixed", i.e. in all valid 4-colorings of the graph when the crossing is removed, they are forced to have the same color. Iterate between all pairs of edges $\{e_1, e_2\}$ that determines a crossing, and check if the two vertices of one of the two edges (e.g. $e_1$) are forced to have the same color in all 4-colorings of the graph in which $e_1$ is removed). Perhaps the problem of checking if two nodes of a 4-colourable (planar) graph are forced to have the same color is already studied?!? – Marzio De Biasi Nov 25 '20 at 11:24
• Do we know the same question on Planar + 1e (a graph obtained from a planar graph by adding one edge)? – Yixin Cao Dec 1 '20 at 3:26