Generalized d'Alembert formula for the telegraph equation

We examine the mixed problem for the telegraph equation with periodic boundary conditions. Using A. P. Khromov's method, we construct a series, which represents the generalized d'Alembert formula for the equation considered. Under minimal conditions for input data, this series gives a generalized solution of the problem. If the criterion of the existence of a (unique) classical solution is fulfilled, then this series also gives a classical solution. The case of a summable potential of the equation is considered. In the case of zero potential, the series mentioned becomes the ordinary d'Alembert formula.