[1] BEHZAD M., HARARY F.: Which directed graphs have a solution?. Math. Slovaca, 27, 1977, 37-42. MR 0469809 | Zbl 0368.05027

[2] BERGE C.: Graphes et hypergraphes. Dunod, Paris 1970. MR 0357173 | Zbl 0213.25702

[3] HARARY F., NORMAN R. Z., CARTWRIGHT D.: Structural models. Wiley, New York 1965. MR 0184874 | Zbl 0139.41503

[4] HEMMINGER R. L.: Line digraphs. Gгaph theory and applications, Springer Verlag, Berlin, 1972, 149-163. MR 0363988 | Zbl 0247.05129

[5] KLERLEIN J. B.: Characterizing line dipseudographs. Proc. 6-th S-E conf. combinatorics, graph theory, and computing, Winnipeg, 1975, 429-442. MR 0396322 | Zbl 0325.05106

[6] RICHARDSON M.: On weakly ordered systems. Bull. Amer. Math. Soc., 52, 1946, 113-116 MR 0014057 | Zbl 0060.06506

[7] ROMANOWICZ Z.: A new proof of Richaгdson theorem. Graphs, hypergraphs aпd block systems, Proc. Symp. Comb. Anal., Zielona Gora, 1976, 227-230.

[8] ШMAДИЧ K.: Ocyщecтвoвaнии peшeний в фaфax, Becтник Лeнинф. Унив. 7, 1976, 88-92.