[1] R. Bellman:
On the boundedness of solutions of nonlinear differential and difference equations. Trans. Amer. Math. Soc. 62 (1974), no. 3, 357-386.
MR 0023996
[2] A. Halanay D. Wexler:
Qualitative Theory of Sampled-Data Systems. (Russian translation), Mir, Moscow (1971).
MR 0352577
[3] I. Zaballa J. M. Gracia:
On difference linear periodic systems I. Homogeneous case. Apl. mat. 28 (1983) 241-248.
MR 0710173