Odd edge coloring of graphs

An edge coloring of a graph G is said to be an odd edge coloring if for each vertex v of G and each color c, the vertex v uses the color c an odd number of times or does not use it at all. In [5], Pyber proved that 4 colors suffice for an odd edge coloring of any simple graph. Recently, some results on this type of colorings of (multi) graphs were successfully applied in solving a problem of facial parity edge coloring [3, 2]. In this paper we present additional results, namely we prove that 6 colors suffice for an odd edge coloring of any loopless connected (multi) graph, provide examples showing that this upper bound is sharp and characterize the family of loopless connected (multi) graphs for which the bound 6 is achieved. We also pose several open problems.