1. The spectrum for lambda-fold 2-perfect 6-cycle systems;Billington;European J. Combin.,1992
2. The spectrum for 2-perfect 6-cycle systems;Lindner;J. Combin. Theory Ser. A,1991
3. C.C. Lindner, C.A. Rodger, Design Theory CRC Press, Boca Raton, FL, 1997, pp. 208.
4. Eine Lösung der beiden Heffterschen Differenzenprobleme;Peltesohn;Compositio Math.,1939