By using the abstract version of Struwe's monotonicity-trick we prove the
existence of a positive solution to the problem (-\Delta)^s u + K u = f(x, u)
in R^N u\in H^s (R^N), K>0 where f(x, t): R^N\times R \rightarrow R is a
Caratheodory function, 1-periodic in x and does not satisfy the
Ambrosetti-Rabinowitz condition
