# Perturbation theorem of Weyl

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

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