# Weyl–von Neumann theorem

In mathematics the Weyl–von Neumann theorem is a result in operator theory due to Hermann Weyl and John von Neumann. It states that, after the addition of a compact operator (Weyl (1909)) or Hilbert–Schmidt operator (von Neumann (1935)) of arbitrarily small norm, a bounded self-adjoint operator or unitary operator on a Hilbert space is conjugate by a unitary operator to a diagonal operator. The results are subsumed in later generalizations for bounded normal operators due to David Berg (1971, compact perturbation) and Dan-Virgil Voiculescu (1979, Hilbert–Schmidt perturbation). The theorem and its generalizations were one of the starting points of operator K-homology, developed first by Larry Brown, Ronald Douglas and Peter Fillmore and, in greater generality, by Gennadi Kasparov.

In 1958 Kuroda showed that the Weyl–von Neumann theorem is also true if the Hilbert–Schmidt class is replaced by any Schatten class Sp with p ≠ 1. For S1, the trace-class operators, the situation is quite different. The Kato–Rosenblum theorem, proved in 1957 using scattering theory, states that if two bounded self-adjoint operators differ by a trace-class operator, then their absolutely continuous parts are unitarily equivalent. In particular if a self-adjoint operator has absolutely continuous spectrum, no perturbation of it by a trace-class operator can be unitarily equivalent to a diagonal operator.

## References

• Conway, John B. (2000), A Course in Operator Theory, Graduate Studies in Mathematics, American Mathematical Society, ISBN 0821820656
• Davidson, Kenneth R. (1996), C*-Algebras by Example, Fields Institute Monographs 6, American Mathematical Society, ISBN 0821805991
• Higson, Nigel; Roe, John (2000), Analytic K-Homology, Oxford University Press, ISBN 0198511760
• Katō, Tosio (1995), Perturbation Theory for Linear Operators, Grundlehren der mathematischen Wissenschaften 132 (2nd ed.), Springer, ISBN 354058661X
• Martin, Mircea; Putinar, Mihai (1989), Lectures on hyponormal operators, Operator theory, advances and applications 39, Birkhäuser Verlag, ISBN 0817623299
• Simon, Barry (2010), Trace Ideals and Their Applications, Mathematical Surveys and Monographs (2nd ed.), American Mathematical Society, ISBN 0821849883
• von Neumann, John (1935), Charakterisierung des Spektrums eines Integraloperators, Actualités Sci. Indust. 229, Hermann
• Weyl, Hermann (1909), "Über beschränkte quadratische Formen, deren Differenz vollstetig ist", Rend. Circolo Mat. Palermo 27: 373–392

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