M. Langer and M. Strauss,
Triple variational principles for self-adjoint operator functions,
J. Funct. Anal. 270 (2016), 2019–2047
Abstract:
For a very general class of unbounded self-adjoint operator function we prove
upper bounds for eigenvalues which lie within arbitrary gaps of the essential spectrum.
These upper bounds are given by triple variations.
Furthermore, we find conditions which imply that a point is in the resolvent set.
For norm resolvent continuous operator functions we show that the variational
inequality becomes an equality.