1. H. Beker and F. Piper: Ciper Systems. Northwood Books, London (1982).
2. Th. Beth, P. Heß, K. Wirl: Kryptographie. Teubner, Stuttgart (1983).
3. G. Cohen and M. Deza: Distances invariantes et L-cliques sur certains demigroupes finis. Math. Sci. Humaines 67, (1979), 49–69.
4. G. Cohen and M. Deza: Some metrical problems on Sn. Annuals of discrete math. 8, (1980), 211–216.
5. J.L. Davison: A result on sums of squares. Cand. Math. Bull. 18(3), (1975), 425–426.