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In mathematics, an equation with an unknown function within an integral. An example is *math.f*(*math.x*) is known and *phiv*(*math.t*) is to be found, given certain conditions on *math.f*. Such equations are useful in solving differential equations.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

In mathematics, the Fredholm integral equation is an integral equation whose solution gives rise to Fredholm theory, the study of Fredholm kernels and Fredholm operators. The integral equation was studied by Ivar Fredholm.
## Equation of the first kind

A homogeneous Fredholm equation of the first kind is written as:## Equation of the second kind

An inhomogeneous Fredholm equation of the second kind is given as## General theory

The general theory underlying the Fredholm equations is known as Fredholm theory. One of the principal results is that the kernel K is a compact operator, known as the Fredholm operator. Compactness may be shown by invoking equicontinuity. As an operator, it has a spectral theory that can be understood in terms of a discrete spectrum of eigenvalues that tend to 0.
## Applications

Fredholm equations arise naturally in the theory of signal processing, most notably as the famous spectral concentration problem popularized by David Slepian.
## See also

## References

- $g(t)=int\_a^b\; K(t,s)f(s),ds$

and the problem is, given the continuous kernel function K(t,s), and the function g(t), to find the function f(s).

If the kernel is a function only of the difference of its arguments, namely $K(t,s)=K(t-s)$, and the limits of integration are $pm\; infty$, then the right hand side of the equation can be rewritten as a convolution of the functions K and f and therefore the solution will be given by

- $f(t)\; =\; mathcal\{F\}\_omega^\{-1\}left[$

where $mathcal\{F\}\_t$ and $mathcal\{F\}\_omega^\{-1\}$ are the direct and inverse Fourier transforms respectively.

- $f(t)=\; lambda\; phi(t)\; -\; int\_a^bK(t,s)phi(s),ds$

Given the kernel K(t,s), and the function $f(t)$, the problem is typically to find the function $phi(t)$. A standard approach to solving this is to use the resolvent formalism; written as a series, the solution is known as the Liouville-Neumann series.

- Integral Equations at EqWorld: The World of Mathematical Equations.
- A.D. Polyanin and A.V. Manzhirov, Handbook of Integral Equations, CRC Press, Boca Raton, 1998. ISBN 0-8493-2876-4
- D. Slepian, "Some comments on Fourier Analysis, uncertainty and modeling", SIAM Review, 1983, Vol. 25, No. 3, 379-393.

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Last updated on Friday September 26, 2008 at 09:58:24 PDT (GMT -0700)

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