Fredholm Solvability of Periodic Neumann Problem for a Linear Telegraph Equation

We investigate the linear telegraph equation $$u_{tt}-u_{xx}+2\mu u_t=f(x,t)$$ with periodic Neumann boundary conditions. We prove that the operator of the problem is modeled as a Fredholm operator of index zero in the scale of Sobolev-type spaces of periodic functions. This result extends to small perturbations of the equation where $\mu$ becomes variable and discontinuous or an additional zero-order term appears. We also show that the solutions to the problem are smoothing.