“…Regarding the first issue, in [19] the authors studied the lack of compactness and gave a precise description of the bubbling phenomena, characterizing the behaviour of the Palais-Smale sequences, in the spirit of classical works [22,21,23,14,15,5,3]. For the strongly indefinite difficulty, in [16,17,18] general functionals with these features are studied by using methods based on a homological approach.…”