“…For one thing, there are many ways to prove the sub-supersolution theorem. For example, Schaefer's fixed point theorem is used in [7,8,11,18,19], pseudomonotone operator theorem is used in [4,5,10], and the idea of monotone iteration is used in [20][21][22] to explore the sub-supersolution theorem of the equation. We must consider the applicability and feasibility of the above methods.…”