Does anyone know where to find something about the perturbation theorem of Weyl, preferably on the internet. The theorem I'm talking about states: let $A$ be a self-adjoint operator on a Hilbert space, and let $B$ be compact with respect to $A$. Then the essential spectrum of $A+B$ is equal to the essential spectrum of $A$. Also, what does this have to do with quantum mechanics? Thanks
You can look at Reed M., Simon B., Methods of Modern Mathematical Physics: Vol. 4 Analysis of Operators, XIII 4.