Hilbert–Schmidt operator

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In mathematics, a Hilbert–Schmidt operator, named for David Hilbert and Erhard Schmidt, is a bounded operator A on a Hilbert space H with finite Hilbert–Schmidt norm

where is the norm of H, an orthonormal basis of H, A* is the adjoint of A, and Tr is the trace of a nonnegative self-adjoint operator.[1][2] Note that the index set need not be countable. This definition is independent of the choice of the basis, and therefore

for and the Schatten norm of for p = 2. In Euclidean space is also called Frobenius norm, named for Ferdinand Georg Frobenius.

The product of two Hilbert–Schmidt operators has finite trace-class norm; therefore, if A and B are two Hilbert–Schmidt operators, the Hilbert–Schmidt inner product can be defined as

The Hilbert–Schmidt operators form a two-sided *-ideal in the Banach algebra of bounded operators on H. They also form a Hilbert space, which can be shown to be naturally isometrically isomorphic to the tensor product of Hilbert spaces

where H is the dual space of H.

The set of Hilbert–Schmidt operators is closed in the norm topology if, and only if, H is finite-dimensional.

An important class of examples is provided by Hilbert–Schmidt integral operators.

Hilbert–Schmidt operators are nuclear operators of order 2, and are therefore compact.

See also[edit]


  1. ^ Moslehian, M. S. "Hilbert–Schmidt Operator (From MathWorld)".
  2. ^ Voitsekhovskii, M. I. (2001) [1994], "H/h047350", in Hazewinkel, Michiel (ed.), Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4