“…For instance, a simple example in tune with previous example is the equationü + q(u, λ) + µh(t, u,u) = 0 when h is 2π-periodic in t andü + q(u) = 0 has nontrivial periodic orbits (see [36] and [4]). More generally, there has been more recently interest about the effect of forced symmetry breaking on the dynamics around some special solutions, invariant manifolds (Galante and Rodrigues [17], Chillingworth and Lauterbach [5], Comanici [7], Parker et al [30] to mention but a few). In these papers, somewhat different techniques are used, linked with equivariant differential geometry.…”