1. Lanford, O., Robinson, D.W.: J. Math. Phys.9, 1120 (1968).
2. Fannes, M.: The entropy density of quasi free states, Preprint July 1972 KUL Leuven Belgium.
3. Kato, T.: Perturbation theory for linear operators. Theorem III.6.10. Berlin-Heidelberg-New York: Springer 1966.