“…The question of the existence of periodic solutions of (1) has received considerable attention in recent years: the existence of classical solutions has been addressed in [4,5,6,7,8,9,10] by using topological methods, whereas the existence of bounded variation solutions has been discussed in [11,12,13] by using non-smooth critical point theory. The advisability of considering bounded variation solutions, besides classical solutions, in order to have a complete picture of the solvability patterns of (1), is already evident for the autonomous equation…”