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Linear differential operators. Part II: Linear differential operators in Hilbert space. With additional material by the author. Translated by E. R. Dawson. English translation edited by W. N. Everitt. (English) Zbl 0227.34020
New York: Frederick Ungar Publishing Co. XV, 352 p. (1968).

34L99 Ordinary differential operators
34-01 Introductory exposition (textbooks, tutorial papers, etc.) pertaining to ordinary differential equations
34G10 Linear differential equations in abstract spaces