  This paper concerns the behavior of time-periodic solutions to 1D dissipative
autonomous semilinear hyperbolic PDEs under the influence of small
time-periodic forcing. We show that the phenomenon of forced frequency locking
happens similarly to the analogous phenomena known for ODEs or parabolic PDEs.
However, the proofs are essentially more difficult than for ODEs or parabolic
PDEs. In particular, non-resonance conditions are needed, which do not have
counterparts in the cases of ODEs or parabolic PDEs. We derive a scalar
equation which answers the main question of forced frequency locking: Which
time shifts of the solution to the unforced equation do survive under which
forcing?
