“…In recent years, with the development of mechanical engineering, control system, computer science, and economics, the existence of solutions of difference equations has attracted wide attention (see [1][2][3][4][5][6]). For example, applying Ricceri variational principle to obtain the existence of multiple solutions [7][8][9], taking the invariant sets of descending flow to prove the existence of sign-changing solutions [10], making the linking theorem to get the existence and multiplicity of periodic solutions [11], and using critical point theory to obtain the existence of homoclinic solutions [12][13][14][15] and heteroclinic solutions [16].…”