1. Sobolev Spaces, Academic Press, New York 1975
2. Lectures on Elliptic Boundary Value Problems, Van Nostrand, Princeton 1965
3. Nonoscillation theory of elliptic equations of order 2n
4. A Kneser Theorem for Higher order Elliptic Equations
5. Partial Differential Equations, Holt, Rinehart and Winston, New York 1969