It was recently shown [{\it R. Mabry}, Bipartite graphs and the four-color theorem, Bull. Inst. Comb. Appl. 14, 119-122 (1995; Zbl 0830.05026)] that the four color theorem is equivalent to the proposition that every planar graph has a factorization into two bipartite graphs. The note under review derives this result as a special case of a formula discovered and proved in [{\it F. Harary, D. F. Hsu} and {\it Z. Miller}, The biparticity of a graph, J. Graph Theory 1, 131-133 (1977; Zbl 0376.05043)].
Reviewer: T.R.Walsh (Montreal)