# Hilbert–Schmidt theorem

In mathematical analysis, the Hilbert–Schmidt theorem, also known as the eigenfunction expansion theorem, is a fundamental result concerning compact, self-adjoint operators on Hilbert spaces. In the theory of partial differential equations, it is very useful in solving elliptic boundary value problems.

## Statement of the theorem

Let (H, 〈 , 〉) be a real or complex Hilbert space and let A : H → H be a bounded, compact, self-adjoint operator. Then there is a sequence of non-zero real eigenvalues λi, i = 1, ..., N, with N equal to the rank of A, such that |λi| is monotonically non-increasing and, if N = +∞,

${\displaystyle \lim _{i\to +\infty }\lambda _{i}=0.}$

Furthermore, if each eigenvalue of A is repeated in the sequence according to its multiplicity, then there exists an orthonormal set φi, i = 1, ..., N, of corresponding eigenfunctions, i.e.

${\displaystyle A\varphi _{i}=\lambda _{i}\varphi _{i}{\mbox{ for }}i=1,\dots ,N.}$

Moreover, the functions φi form an orthonormal basis for the range of A and A can be written as

${\displaystyle Au=\sum _{i=1}^{N}\lambda _{i}\langle \varphi _{i},u\rangle \varphi _{i}{\mbox{ for all }}u\in H.}$

## 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. 356. ISBN 0-387-00444-0. (Theorem 8.94)