“…In the past decades, periodic problems involving the relativistic forced pendulum differential equation for the continuous case T = R were studied by many authors, see [3,4,8,13,17,18]. In particular, the works [3,18] are concerned with the so-called solvability set, that is, the set I(p 0 ) of values of s for which (1) has at least one T -periodic solution. We remark that problem (1) is 2πperiodic and, consequently, if x is a T -periodic solution then x + 2kπ is also a T -periodic solution for all k ∈ Z.…”