Flaška, V.; Ježek, J.; Kepka, T.; Kortelainen, J. (2007). Transitive Closures of Binary Relations I(PDF). Prague: School of Mathematics - Physics Charles University. p. 1. Arquivado dende o orixinal(PDF) o 2013-11-02. Lemma 1.1 (iv).
Flaška, V.; Ježek, J.; Kepka, T.; Kortelainen, J. (2007). Transitive Closures of Binary Relations I(PDF). Prague: School of Mathematics - Physics Charles University. p. 1. Arquivado dende o orixinal(PDF) o 2013-11-02. Lemma 1.1 (iv).