“…Since 2003, the critical point theory has been extensively applied to study the existence and multiplicity of periodic solutions and boundary value problems to difference equations involving a p-Laplacian operator (more generally, ϕ-Laplacian operator) and higher order difference equations (cf. [3,20,21,22,27,32,51,67,72,84,114]. In this section, we only list several results in the literature.…”