# Hilbert–Schmidt integral operator

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In mathematics, a Hilbert–Schmidt integral operator is a type of integral transform. Specifically, given a domain (an open and connected set) Ω in n-dimensional Euclidean space Rn, a Hilbert–Schmidt kernel is a function k : Ω × Ω → C with

${\displaystyle \int _{\Omega }\int _{\Omega }|k(x,y)|^{2}\,dx\,dy<\infty }$

(that is, the L2(Ω×Ω; C) norm of k is finite), and the associated Hilbert–Schmidt integral operator is the operator K : L2(Ω; C) → L2(Ω; C) given by

${\displaystyle (Ku)(x)=\int _{\Omega }k(x,y)u(y)\,dy.}$

Then K is a Hilbert–Schmidt operator with Hilbert–Schmidt norm

${\displaystyle \Vert K\Vert _{\mathrm {HS} }=\Vert k\Vert _{L^{2}}.}$

Hilbert–Schmidt integral operators are both continuous (and hence bounded) and compact (as with all Hilbert–Schmidt operators).

The concept of a Hilbert–Schmidt operator may be extended to any locally compact Hausdorff spaces. Specifically, let X be a locally compact Hausdorff space equipped with a positive Borel measure. Suppose further that L2(X) is a separable Hilbert space. The above condition on the kernel k on Rn can be interpreted as demanding k belong to L2(X × X). Then the operator

${\displaystyle (Kf)(x)=\int _{X}k(x,y)f(y)\,dy}$

is compact. If

${\displaystyle k(x,y)={\overline {k(y,x)}}}$

then K is also self-adjoint and so the spectral theorem applies. This is one of the fundamental constructions of such operators, which often reduces problems about infinite-dimensional vector spaces to questions about well-understood finite-dimensional eigenspaces. See Chapter 2 of the book by Bump in the references for examples.

## References

• Renardy, Michael; Rogers, Robert C. (2004). An introduction to partial differential equations. Texts in Applied Mathematics 13 (Second ed.). New York: Springer-Verlag. p. 262. ISBN 0-387-00444-0. (Sections 8.1 and 8.5)
• Bump, Daniel (1998). Automorphic Forms and Representations. Cambridge Studies in Advanced Mathematics. 55. Cambridge: Cambridge University Press. p. 168. ISBN 0-521-65818-7.