1. Hörmander, L., On the theory of general partial differential operators, Acta Math., Vol. 94, 1955, pp. 161–248.
2. Hölmgren, E., Über Systeme von linear en partiellen Differentialgleichungen. Öfversigt Kongl. Vetens.-Akad. Förh., Vol. 58, 1901, pp. 91–103.
3. John, F., On linear partial differential equations with analytic coefficients. Unique Continuation of Data, Comm. Pure Appl. Math., Vol. 2, 1949, pp. 209–253.
4. Gårding, L., Linear hyperbolic partial differential equations with constant coefficients, Acta Math., Vol. 85, 1950, pp. 1–62.
5. Ehrenpreis, L., Cauchy’s problem for linear differential equations with constant coefficients, Proc. Nat. Acad. Sci. U.S.A., Vol. 42, 1956, pp. 642–646.