1. On minimal regular digraphs with given girth;Behzad;Fund. Math.,1970
2. Extremal Graph Theory;Bollobás,1978
3. J.A. Bondy, Counting techniques in graph theory, in preparation.
4. On minimal digraphs with given girth;Caccetta,1978
5. Short cycles in directed graphs;Chvátal;J. Combin. Theory Ser. B,1983