1. W. Ahrens, Mathematische Unterhaltungen und Spiele. Vol. 2, Teubner, Leipzig, 1918.
2. E. F. Assmus, Jr. and H. F. Mattson, Jr., Steiner systems and perfect codes, University of North Carolina Institute of Statistics Mimeo Series, No. 484-1, 1966.
3. \bysame, Research problem, J. Combinatorial Theory 3 (1967), 307.
4. S. Bays, Une question de Cayley relative au problème des triades de Steiner, Enseignement Math. 19 (1917), 57-67.
5. On the construction of balanced incomplete block designs;Bose, R. C.;Ann. Eugenics,1939