Grünbaum, B. (1970), "A problem in graph coloring", American Mathematical Monthly, 77 (10), Mathematical Association of America: 1088–1092, doi:10.2307/2316101, JSTOR2316101.
Grünbaum, B. (1970), "A problem in graph coloring", American Mathematical Monthly, 77 (10), Mathematical Association of America: 1088–1092, doi:10.2307/2316101, JSTOR2316101.