In mathematics, an integral equation is an equation in which an unknown function appears under an integral sign. There is a close connection between differential and integral equations, and some problems may be formulated either way. See, for example, Maxwell's equations.
The most basic type of integral equation is a Fredholm equation of the first type:
The notation follows Arfken. Here φ is an unknown function, f is a known function, and K is another known function of two variables, often called the kernel function. Note that the limits of integration are constant; this is what characterizes a Fredholm equation.
If the unknown function occurs both inside and outside of the integral, it is known as a Fredholm equation of the second type:
If one limit of integration is variable, it is called a Volterra equation. Thus Volterra equations of the first and second types, respectively, would appear as:
In all of the above, if the known function f is identically zero, it is called a homogeneous integral equation. If f is nonzero, it is called an inhomogeneous integral equation.
Numerical Solution 
It is worth noting that Integral Equations often do not have an analytical solution, and must be solved numerically. An example of this is evaluating the EFIE or MFIE equations over an arbitrarily shaped object in an electromagnetic scattering problem.
One method to solve numerically requires discretizing variables and replacing integral by a quadrature rule
for . Then we have a equations and variables system. By solving it we get the value of the variables .
Integral equations are classified according to three different dichotomies, creating eight different kinds:
- Limits of integration
- both fixed: Fredholm equation
- one variable: Volterra equation
- Placement of unknown function
- only inside integral: first kind
- both inside and outside integral: second kind
- Nature of known function f
- identically zero: homogeneous
- not identically zero: inhomogeneous
Integral equations are important in many applications. Problems in which integral equations are encountered include radiative energy transfer and the oscillation of a string, membrane, or axle. Oscillation problems may also be solved as differential equations.
Both Fredholm and Volterra equations are linear integral equations, due to the linear behaviour of φ(x) under the integral. A nonlinear Volterra integral equation has the general form:
where F is a known function.
Wiener-Hopf integral equations 
Originally, such equations were studied in connection with problems in radiative transfer, and more recently, they have been related to the solution of boundary integral equations for planar problems in which the boundary is only piecewise smooth.
Integral equations as a generalization of eigenvalue equations 
where is a matrix, is one of its eigenvectors, and is the associated eigenvalue.
Taking the continuum limit, by replacing the discrete indices and with continuous variables and , gives
where the sum over has been replaced by an integral over and the matrix and vector have been replaced by the 'kernel' and the eigenfunction . (The limits on the integral are fixed, analogously to the limits on the sum over .) This gives a linear homogeneous Fredholm equation of the second type.
In general, can be a distribution, rather than a function in the strict sense. If the distribution has support only at the point , then the integral equation reduces to a differential eigenfunction equation.
- Kendall E. Atkinson The Numerical Solution of integral Equations of the Second Kind. Cambridge Monographs on Applied and Computational Mathematics, 1997.
- George Arfken and Hans Weber. Mathematical Methods for Physicists. Harcourt/Academic Press, 2000.
- Andrei D. Polyanin and Alexander V. Manzhirov Handbook of Integral Equations. CRC Press, Boca Raton, 1998. ISBN 0-8493-2876-4.
- E. T. Whittaker and G. N. Watson. A Course of Modern Analysis Cambridge Mathematical Library.
- M. Krasnov, A. Kiselev, G. Makarenko, Problems and Exercises in Integral Equations, Mir Publishers, Moscow, 1971
- Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (2007). "Chapter 19. Integral Equations and Inverse Theory". Numerical Recipes: The Art of Scientific Computing (3rd ed.). New York: Cambridge University Press. ISBN 978-0-521-88068-8.