“…We refer here to some works using the upper and lower solutions method, e.g., see [3,10,11,13,14,15,18,20,21,24,25,28,29] for finite interval problems, and [1,5,7,27] for infinite interval problems. Discrete infinite interval problems have also been studied by several other methods in [2,4,6,8,9,12,16,17,19,21,22,26].…”