On the 4-color theorem for signed graphs
Kardo\v{s}, Franti\v{s}ek, Jonathan Narboni · arXiv (Cornell University) · 2019
There are several ways to generalize graph coloring to signed graphs. M\'a\v{c}ajov\'a, Raspaud and \v{S}koviera introduced one of them and conjectured that in this setting, for signed planar graphs four colors are always enough, generalising thereby The Four Color Theorem. We disprove the conjecture.