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P. D. Lax, On Cauchy's problem for hyperbolic equations and the differentiability of solutions of elliptic equations, Comm. Pure Appl. Math. 8 (1955), 615-633. MR 0078558 (17:1212c)

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L. Hörmander, On the theory of general partial differential operators, Acta Math. 94 (1955), 161-248. MR 0076151 (17:853d)

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N. I. Achiezer and I. M. Glasman, Theorie der linearen Operatoren im Hilbert-Raum, Akademie-Verlag, Berlin, 1954. MR 0066560 (16:596f)

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G. C. Rota, Extension theory of differential operators. I, Comm. Pure Appl. Math. 11 (1958), 23-65. MR 0096852 (20:3334)

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I. C. Gohberg and M. G. Krein, Systems of integral equations on a half line with kernels depending on the difference of arguments, Amer. Math. Soc. Transl. (2) 14 (1960), 217-287. MR 0113114 (22:3954)

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F. Riesz and B. Sz. Nagy, Functional analysis, Ungar, New York, 1955. MR 0071727 (17:175i)