“…Among these approaches, the critical point theory seems to be a powerful tool to deal with this problem (see [5,[7][8][9]). However, compared to the boundary value problems of lower order difference equations ( [6,8,[10][11][12][13]), the study of boundary value problems of higher order difference equations is relatively rare (see [9,14,15]), especially the works by using the critical point theory [16]. For the background on difference equations, we refer to [17].…”