“…Since the behaviour of a discrete system is sometimes sharply different to the behaviour of the corresponding continuous system (see, for example, [1,16,17]), many scholars have investigated discrete Hamiltonian systems for the disconjugacy, boundary value problems, oscillations and asymptotic behaviour of (1.1) [3,5,7,8,11]. Only very few papers deal with the existence of periodic solutions even to general difference equations, see for example [2,9,16,17].…”