1. Adams, R.A.: Sobolev Spaces. New York London: Academic Press 1975
2. Ancona, A.: On strong barriers and an inequality of Hardy for domains in R n . J. Lond. Math. Soc.34, 274–290 (1986)
3. Ciarlet, P.G.: The finite element method for elliptic equations. Amsterdam: North Holland 1978
4. Davies, E.B.: One-parameter semigroups. New York London: Academic Press 1980
5. Davies, E.B.: Some norm bounds and quadratic form inequalities for Schrödinger operators. J. Oper. Theory9, 147–162 (1993)