Balanced chromatic number and Hadwiger-like conjectures

Motivated by different characterizations of planar graphs and the 4-Color Theorem, several structural results concerning graphs of high chromatic number have been obtained. Toward strengthening some of these results, we consider the balanced chromatic number , χ b ( G ˆ ) , of a signed graph G ˆ . This is the minimum number of parts into which the vertices of a signed graph can be partitioned so that none of the parts induces a negative cycle. This extends the notion of the chromatic number of a graph since χ ( G ) = χ b ( G ̃ ) , where G ̃ denotes the signed graph obtained from G by replacing each edge with a pair of (parallel) positive and negative edges. We introduce a signed version of Hadwiger’s conjecture as follows. Conjecture . If a signed graph G ˆ has no negative loop and no K ̃ t -minor, then its balanced chromatic number is at most t − 1 . We prove that this conjecture is, in fact, equivalent to Hadwiger’s conjecture and show its relation to the odd Hadwiger Conjecture. Motivated by these results, we also consider the relation between subdivisions and balanced chromatic number. We prove that if ( G , σ ) has no negative loop and no K ̃ t -subdivision, then it admits a balanced 79 2 t 2 -coloring. This qualitatively generalizes a result of Kawarabayashi (2013) on totally odd subdivisions. Finally, following supportive results in the literature on the fractional variant of Hadwiger’s conjecture, we show that the fractional balanced chromatic number of any signed graph with no positive loop and no K ̃ t -minor is at most 2 t − 2 .