1. M. Reed and B. Simon, Methods of Modern Mathematical Physics, IV: Analysis of Operators, Academic Press, New York, 1978.
2. W. Allegretto, Principal eigenvalues for indefinite-weight elliptic problems in $\mathbbR^n$, Proc. Amer. Math. Soc. 116 (1992), 701--706.
3. W. Arendt and C. J. K. Batty, Exponential stability of a diffusion equation with absorption, Differential Integral Equations 6 (1993), 1009--1024.
4. --. --. --. --., The spectral function and principal eigenvalues for Schrödinger operators, Potential Anal. 7 (1997), 415--436.
5. T. Aubin, Nonlinear Analysis on Manifolds: Monge-Ampère Equations, Grundlehren Math. Wiss. 252, Springer, New York, 1982.