The method of characteristics is used to construct the fundamental solution to a system of second‐order partial differential equations. Conditions that the coefficients have to satisfy to preserve the hyperbolicity are determined and the influence of lower‐order terms is indicated. As an example, the Proca wave equation in an external field is analyzed.

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This section is essentially a further development of a paper by the author;
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Velo and Zwanziger, Ref. 3, Appendix A.
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