A perturbed wave equation amenable to exact reductionIn [2] we have considered the problem of finding T -periodic solutions∂ x 2 is the D'Alembert wave operator while F is a Nemitski Lipschitz operator. To produce an exact finite reduction in the wave equation we needed the inverse operator of the D'Alembertian. To arrive to this scope we first expanded v ∈ H in Fourier series and then we laid down the formal explicit component-wise inverse of :where k ∈ Z n , k = 0. However, this formula is not valid in general, i.e., the D'Alembert operator does not admit a bounded inverse in H both with rational and irrational periods T . This could be overcome by considering numbers of constant typeas in [4]; this, however, is not compatible with the contractivity needed for the reduction procedure. We have instead adopted the standard approach by Rabinowitz [5], i.e., we considered the vibrating string (n = 1), weThe online version of the original article can be found under