An Application of Graph Theory to Algebra
Richard G. Swan · Proceedings of the American Mathematical Society · 1963
For an interesting discussion of this result, see [4], 3 The connection between Theorem 1 and graph theory was also pointed out by M. Schutzenberger [3, p. 167].A proof using his idea was recently worked out by C. Y. Chao and Schutzenberger.Their proof seems to be quite different from mine.