1. L. D. Cummings, ?On a method of comparison for triple systems,? Trans. Am. Math. Soc.,15, 311?327 (1914).
2. I. P. Neporozhnev and A. Ya. Petrenik, ?A constructive enumeration of systems of groups of pairs and tabulated systems of Steiner triples,? in: Combinatorial Analysis [in Russian], No. 2, Moscow State Univ. (1972), pp. 17?37.
3. M. Reiss, ?Über eine Steinerhe combinatorische Aufgabe welche in 45-sten Bande dieses Journals, Seite 181, gestellt worden ist,? J. Reine Angew. Math.,56, 326?344 (1859).
4. H. Hanani, ?On quadruple systems,? Can. J. Math.,12, 145?157 (1960).
5. N. P. Korovina, ?On certain classes of systems of groups of pairs,? in: Combinatorial Analysis [in Russian], No. 3, Moscow State Univ., Moscow (1974), pp. 43?48.