Breather solutions for a semilinear Klein–Gordon equation on a periodic metric graph

We consider the nonlinear Klein-Gordon equation $$ \partial_t^2 u(x,t) - \partial_x^2 u(x,t) + \alpha u(x,t) = \pm |u(x,t)|^{p-1}u(x,t) $$ on a periodic metric graph (necklace graph) for $p > 1$ with Kirchhoff conditions at the vertices. Under suitable assumptions on the frequency we prove the existence and regularity of infinitely many spatially localized time-periodic solutions (breathers) by variational methods. We compare our results with previous results obtained via spatial dynamics and center manifold techniques. Moreover, we deduce regularity properties of the solutions and show that they are weak solutions of the corresponding initial value problem. Our approach relies on the existence of critical points for indefinite functionals, the concentration compactness principle, and the proper set-up of a functional analytic framework. Compared to earlier work for breathers using variational techniques, a major improvement of embedding properties has been achieved. This allows in particular to avoid all restrictions on the exponent $p > 1$ and to achieve higher regularity.