We present here a refutation of a paper by Bernstein and Book dealing with the perturbation theory and the stability problem of the Primakoff blast waves, which are a degenerate class of self-similar flows characterized by a state of homologous expansion. The main error in Bernstein and Book's treatment consists in their neglect of the entropy perturbations that necessarily arise at the shock front as a result of the Rankine-Hugoniot continuity relations. Their decomposition into normal modes can at most accommodate the discrete part of the spectrum, which in the present case is essentially a continuous spectrum. The radial discrete spectrum that they derive is for the most part made up of spurious modes, devoid of physical meaning.
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B. Gaffet (1984) studied this question.