Volterra integral equation explained

Volterra integral equation should not be confused with Product integral.

In mathematics, the Volterra integral equations are a special type of integral equations.^[1] They are divided into two groups referred to as the first and the second kind.

A linear Volterra equation of the first kind is

f(t) = \int ^{t} _{a} K(t,s)x(s)ds

where f is a given function and x is an unknown function to be solved for. A linear Volterra equation of the second kind is

x(t) = f(t) + \int ^{t} _{a} K(t,s)x(s)ds.

In operator theory, and in Fredholm theory, the corresponding operators are called Volterra operators. A useful method to solve such equations, the Adomian decomposition method, is due to George Adomian.

A linear Volterra integral equation is a convolution equation if

x(t) = f(t) + \int ^{t} _{t _{0}} K(t - s)x(s)ds.

The function

K in the integral is called the kernel. Such equations can be analyzed and solved by means of Laplace transform techniques.

For a weakly singular kernel of the form

K(t,s) = (t ^{2-s} ^{2)} ^{-\alpha} with 0 <\alpha< 1 , Volterra integral equation of the first kind can conveniently be transformed into a classical Abel integral equation.

The Volterra integral equations were introduced by Vito Volterra and then studied by Traian Lalescu in his 1908 thesis, Sur les équations de Volterra, written under the direction of Émile Picard. In 1911, Lalescu wrote the first book ever on integral equations.

Volterra integral equations find application in demography as Lotka's integral equation,^[2] the study of viscoelastic materials,in actuarial science through the renewal equation,^[3] and in fluid mechanics to describe the flow behavior near finite-sized boundaries.^{[4] [5]}

Conversion of Volterra equation of the first kind to the second kind

A linear Volterra equation of the first kind can always be reduced to a linear Volterra equation of the second kind, assuming that

K(t,t) ≠ 0 . Taking the derivative of the first kind Volterra equation gives us:$$=\; \backslash int\_^x(s)ds\; +\; K(t,t)x(t)$$Dividing through by K(t,t) yields:$$x(t)\; =\; -\; \backslash int\_^x(s)ds$$Defining $\backslash widetilde(t)\; =$ and $\backslash widetilde(t,s)\; =\; -$ completes the transformation of the first kind equation into a linear Volterra equation of the second kind.

Numerical solution using trapezoidal rule

A standard method for computing the numerical solution of a linear Volterra equation of the second kind is the trapezoidal rule, which for equally-spaced subintervals

\Delta x is given by:$$\backslash int\_^f(x)dx\; \backslash approx\; \backslash left[f(x\_\{0\})\; +\; 2\backslash sum\_\{i=1\}^\{n-1\}\; f(x\_\{i\})\; +\; f(x\_\{n\})\; \backslash right\; ]$$Assuming equal spacing for the subintervals, the integral component of the Volterra equation may be approximated by:$$\backslash int\_^K(t,s)x(s)ds\; \backslash approx\; \backslash left[K(t,s\_\{0\})x(s\_\{0\})\; +\; 2K(t,s\_\{1\})x(s\_\{1\})\; +\; \backslash cdots\; +\; 2K(t,s\_\{n-1\})x(s\_\{n-1\})\; +\; K(t,s\_\{n\})x(s\_\{n\})\; \backslash right\; ]$$Defining x _{i} = x(s _{i} ) , f _{i} = f(t _{i} ) , and K _{ij} = K(t _{i} ,s _{j} ) , we have the system of linear equations:This is equivalent to the matrix equation:$$x\; =\; f\; +\; Mx\; \backslash implies\; x\; =\; (I-M)^f$$For well-behaved kernels, the trapezoidal rule tends to work well.

Application: Ruin theory

One area where Volterra integral equations appear is in ruin theory, the study of the risk of insolvency in actuarial science. The objective is to quantify the probability of ruin

\psi(u) = P [\tau(u) < inf ty] , where u is the initial surplus and \tau(u) is the time of ruin. In the classical model of ruin theory, the net cash position X _{t} is a function of the initial surplus, premium income earned at rate c , and outgoing claims \xi :$$X\_\; =\; u\; +\; ct\; -\; \backslash sum\_^\backslash xi\_,\; \backslash quad\; t\; \backslash geq\; 0$$where N _{t} is a Poisson process for the number of claims with intensity λ . Under these circumstances, the ruin probability may be represented by a Volterra integral equation of the form^[6] :$$\backslash psi(u)\; =\; \backslash int\_^S(x)dx\; +\; \backslash int\_^\backslash psi(u-x)S(x)dx$$where S( ⋅ ) is the survival function of the claims distribution.

See also

• Fredholm integral equation
• Integral equation
• Integro-differential equation

References

1. Book: Polyanin, Andrei D.. Handbook of Integral Equations. Manzhirov. Alexander V.. Chapman and Hall/CRC. 2008. 978-1584885078. 2nd. Boca Raton, FL.
2. Book: Inaba, Hisashi . The Stable Population Model . Age-Structured Population Dynamics in Demography and Epidemiology . 1–74 . Singapore . Springer . 2017 . 978-981-10-0187-1 . 10.1007/978-981-10-0188-8_1 .
3. Book: Brunner, Hermann. Volterra Integral Equations: An Introduction to Theory and Applications. Cambridge University Press. 2017. 978-1107098725. Cambridge Monographs on Applied and Computational Mathematics. Cambridge, UK.
4. Daddi-Moussa-Ider. A.. Vilfan. A.. Ramin Golestanian. Golestanian. R.. Diffusiophoretic propulsion of an isotropic active colloidal particle near a finite-sized disk embedded in a planar fluid–fluid interface. Journal of Fluid Mechanics. 6 April 2022. 940. A12. 10.1017/jfm.2022.232. 2109.14437.
5. Daddi-Moussa-Ider. A.. Lisicki. M.. Hartmut Löwen. Löwen. H.. Menzel. A. M.. Dynamics of a microswimmer–microplatelet composite. Physics of Fluids. 5 February 2020. 32. 2. 021902. 10.1063/1.5142054. 2001.06646.
6. Web site: Lecture Notes on Risk Theory. February 20, 2010. School of Mathematics, Statistics and Actuarial Science. University of Kent. 17-22.

Further reading

• Traian Lalescu, Introduction à la théorie des équations intégrales. Avec une préface de É. Picard, Paris: A. Hermann et Fils, 1912. VII + 152 pp.
  • • • Integral Equations: Exact Solutions at EqWorld: The World of Mathematical Equations
• Book: Press . WH . Teukolsky . SA . Vetterling . WT . Flannery . BP . 2007 . Numerical Recipes: The Art of Scientific Computing . 3rd . Cambridge University Press . New York . 978-0-521-88068-8 . Section 19.2. Volterra Equations . http://apps.nrbook.com/empanel/index.html#pg=992.

External links

• IntEQ: a Python package for numerically solving Volterra integral equations