1. The product formula for semigroups defined by Friedrichs extensions;Faris;Pacific J. Math.,1967
2. Perturbations of the Schroedinger equation by potentials with small support, semigroup product formulas, and applications to quantum mechanics;Friedman,1971
3. Perturbation Theory for Linear Operators;Kato,1966
4. Quantum Mechanics;Landau,1965
5. On elliptic partial differential equations;Nirenberg;Ann. Scuola Norm. Sup. Pisa,1959