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 and an orthonormal basis of H for an index set . 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 . 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.