1. Six-rings in minimal five-color maps;Bernhart;Am. J. Math.,1947
2. Topics in graph theory related to the five color conjecture;Bernhart,1974
3. Some remarks on open sets of colorings;Bernhart,1974
4. The reducibility of maps;Birkhoff;Amer. J. Math.,1913
5. Chromatic polynomials;Birkhoff;Trans. Amer. Math. Soc.,1946