“…On the one hand, with the sharp development of difference equations' studies, difference equations have been widely used as mathematical models to describe real life situations in probability theory, matrix theory, electrical circuit analysis, combinatorial analysis, number theory, psychology and sociology, and etc. On the other hand, the critical point theory has been widely used to study the existence of periodic solutions, subharmonic solutions and boundary value problems to difference equations, for example, Guo, Yu and Zhou [16][17][18]20] have done many excellent work on it by using critical point theory. In view of above reasons, we will devote ourselves to looking for periodic solutions having minimal period for (1.1).…”