Abstract wave equations with generalized Wentzell boundary conditions

We introduce a general framework which allows to verify if abstract wave equations with generalized Wentzell boundary conditions are well-posed, i.e., are governed by a cosine family. As an example we study wave equations for second order differential operators on C[0,1] with non-local Wentzell-type boundary conditions. Moreover, in Appendix A we give a perturbation result for sine and cosine families.