Some generalizations of radial Bremmer series

In this paper we address the problem of finding a solution of the wave equation with a non-constant propagation velocity whereby the attention is focussed on the case of spherical waves. By a subdivision of the medium into spherical shells one particular so-called Bremmer series is derived and it is shown that other Bremmer series are possible too. It turns out that each of these series has a different convergence criterion, the fulfillment of which depends on the physical situation under consideration. Hence in each physical situation there is some freedom in choosing an appropriate Bremmer series.