“…In the present paper, we show a new abstract theorems, see Theorems 2.1, 4.4 and 6.1, which can be used to prove the existence of solution for (1.1) and related problems. These theorems are totally different from that proved in [1] and their proofs are very simple, however Theorem 4.4 permits to show the existence of multiple solutions for a class of zero mass problem that is a new result for this class of problem. The present article completes the study made in [1], in the sense that we can consider other class of problems, for example, our approach can be applied to study a version of Berestycki-Lions type problems for elliptic system, which were not considered until moment in the literature, see Section 6.…”