# Integral Equations

Also found in: Dictionary.
Related to Integral Equations: Fredholm integral equations, Differential equations
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Integral Equations

equations containing unknown functions under the integral sign. Numerous problems of physics and mathematical physics lead to various types of integral equations. Suppose, for example, we are required to obtain with the help of an optical instrument an image of a linear object A occupying the interval 0 ≤ xl on the x-axis, where the illumination of the object is characterized by a density function u(x). The image B is a certain interval on another axis x1. By a suitable choice of origin and unit of length the latter interval can be made to coincide with the interval 0 ≤ x1l. If an infinitesimal part (x,x + Δ x) of the object A causes an illumination of the image B with density K(xi,x) u(x) dx, where the function K(x1,x) is determined by the properties of the optical instrument, then the total illumination of the image will have density Depending on whether we wish to obtain a given illumination v(x,1) of the image, or an “accurate” photographic image [v(x) = ku(x), where the constant k is not fixed in advance], or, finally, a specified difference of the illumination of A and B [ u(x)— v(x) = f(x)], we arrive at various integral equations for the function u(x): In general, an integral equation of the form is called a linear integral equation of the first kind, whereas an equation of the form is called a linear integral equation of the second kind, or a Fredholm equation [when f(x) ≡ 0 it is called a homogeneous Fredholm equation]. A Fredholm equation usually involves a parameter λ: In all equations, the function

K(x,j) (axb, ayb)

the kernel of the integral equation, is given as is the function f(x) (a ≤ xb); the unknown function is u(x) (axb).

The functions K(x, y), f(x), and u(x) and the parameter λ can take on real or complex values. In the particular case when the kernel K(x, y) vanishes for y > x, we obtain the Volterra equation: An integral equation is called singular if at least one of the limits of integration is infinite or if the kernel K(x, y) becomes infinite at one or several points of the square axb, ayb or on a certain curve. Integral equations can involve functions of several variables. A relevant example is furnished by the equation Nonlinear integral equations are also considered, for example, equations of the form Linear integral equations of the second kind are solved by the following methods. (1) The solution u(x) is obtained in the form of a power series in λ (convergent in a certain circle |λ| < K) with coefficients that depend on x (the Volterra-Neumann method). (2) The solution u(x), for values of λ for which it exists, is expressed in terms of certain entire functions of λ (the Fredholm method). (3) In the case when the kernel is symmetric, that is, K(x, y) ≡ K(y,x), the solution u(x) is expressed in the form of a series of orthogonal functions uk(x), which are nonzero solutions of the corresponding homogeneous equation (the latter has nontrivial solutions only for certain values λ = λkk = 1, 2, …, of the parameter λ) (the Hilbert-Schmidt method). (4) In certain exceptional cases, a solution is comparatively easily obtained with the help of the Laplace transform. (5) In the case when (the case of the so-called degenerate kernel), the determination of u(x) reduces to the solution of a system of algebraic equations. Approximate solutions may be obtained, either by applying some formula for numerical integration to abK(x,t) u(t) dt or by substituting for the given kernel K(x,y). some degenerate kernel that differs slightly from K(x,y). Boundary value problems for ordinary and partial differential equations often reduce to integral equations; such a reduction is of theoretical and practical value.

### REFERENCES

Smirnov, V. I. Kurs vysshei matematiki, 3rd ed., vol. 4. Moscow, 1957.
Petrovskii, I. G. Lektsii po teorii integral’nykh uravnenii, 3rd ed. Moscow, 1965.
Kantorovich, L. V., and V. I. Krylov. Priblizhennye melody vysshego analiza, 5th ed. Leningrad-Moscow, 1962.

D. A. VASIL’KOV

References in periodicals archive ?
This "photonic calculus" would work by encoding parameters into the properties of an incoming electromagnetic wave and sending it through a metamaterial device; once inside, the device's unique structure would manipulate the wave in such a way that it would exit encoded with the solution to a pre-set integral equation for that arbitrary input.
In particular, can the results be extended to the case of weakly singular Volterra integral equations? We hope to answer these questions elsewhere.
If the lower limit of integration is constant and the upper one is variable we are in the case of Volterra integral equations which is our subject.
Abbas and colleagues present detailed results about the existence and stability of various classes of fractional differential and integral equations and inclusions involving the Caputo fractional derivative and the Hadamard fractional integral.
The system of algebraic equations, with the help of which the integral equations are solved, for equation (6) has a much smaller order.
In order to solve the problem the approach [7, 8] based on joint application of the boundary integral equations method and time Fourier transform is used 
For the problem with periodic boundary conditions the integral equation with Hilbert kernel is used :
In this section, we use the proposed method in two-dimensional linear fuzzy Fredholm integral equations for solving two examples.
The fractional differential transform method has recently been developed for solving the differential and integral equations. For example, in , FDTM is applied for fractional differential equations and in  it is used for fractional integro-differential equations.
Tricomi, Integral Equations, Interscience, New York, NY, USA, 1957.
Mingarelli, Volterra-Stieltjes Integral Equations and Generalized Ordinary Differential Expressions, vol.

Site: Follow: Share:
Open / Close