  The goal of this work is to study the existence of quasi-periodic solutions
in time to nonlinear beam equations with a multiplicative potential. The
nonlinearities are required to only finitely differentiable and the frequency
is along a pre-assigned direction. The result holds on any compact Lie group or
homogenous manifold with respect to a compact Lie group, which includes the
standard torus $\mathbf{T}^{d}$, the special orthogonal group $SO(d)$, the
special unitary group $SU(d)$, the spheres $S^d$ and the real and complex
Grassmannians. The proof is based on a differentiable Nash-Moser iteration
scheme.

Dynamical Systems