1. The identity of weak and strong extensions of differential operators
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3. 2nd Edit. 1957.
4. (A) Lectures on hyperbolic equations with variable coefficients, Institute for Advanced Study, Princeton, Fall, 1952.
5. (B) On linear hyperbolic differential equations with variable coefficients on a vector space, Contributions to the Theory of Partial Differential Equations, Ann. Math. Studies, No. 33, Princeton University Press, 1954, pp. 201–210.