1. Lectures on Elliptic Boundary Value Problems (Van Nostrand, Princeton, N. J., 1965).
2. Multiparameter Eigenvalue Problems, Vol. 1 (Academic, New York, 1972).
3. Asymptotic behavior of the spectrum of differential equations
4. Two-parameter Eigenvalue Problems in Ordinary Differential Equations (Longman, Harlow, 1991).