2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/161853We show that we cannot avoid the existence of at least one directed circuit of length less than or equal to (n/r) in a digraph on n vertices with out-degree greater than or equal to r. This is well-known Caccetta-Haggkvist problem.23 pages. A proof of Seymour second neighborhood conjecture is addedGeneral Mathematicstext