Appel, K.; Haken, W. Every planar map is four colorable. (English) Zbl 0331.05106 Bull. Am. Math. Soc. 82, 711-712 (1976). MSC: 05C15 PDFBibTeX XMLCite \textit{K. Appel} and \textit{W. Haken}, Bull. Am. Math. Soc. 82, 711--712 (1976; Zbl 0331.05106) Full Text: DOI
McLaughlin, T. G. Co-immune retraceable sets. (English) Zbl 0221.02026 Bull. Am. Math. Soc. 71, 523-525 (1965). MSC: 03D55 PDFBibTeX XMLCite \textit{T. G. McLaughlin}, Bull. Am. Math. Soc. 71, 523--525 (1965; Zbl 0221.02026) Full Text: DOI
Lyndon, R. C. Properties preserved under algebraic constructions. (English) Zbl 0087.00903 Bull. Am. Math. Soc. 65, 287-299 (1959). PDFBibTeX XMLCite \textit{R. C. Lyndon}, Bull. Am. Math. Soc. 65, 287--299 (1959; Zbl 0087.00903) Full Text: DOI