Fundamental solution

Summary

In mathematics, a fundamental solution for a linear partial differential operator L is a formulation in the language of distribution theory of the older idea of a Green's function (although unlike Green's functions, fundamental solutions do not address boundary conditions).

In terms of the Dirac delta "function" δ(x), a fundamental solution F is a solution of the inhomogeneous equation

LF = δ(x).

Here F is a priori only assumed to be a distribution.

This concept has long been utilized for the Laplacian in two and three dimensions. It was investigated for all dimensions for the Laplacian by Marcel Riesz.

The existence of a fundamental solution for any operator with constant coefficients — the most important case, directly linked to the possibility of using convolution to solve an arbitrary right hand side — was shown by Bernard Malgrange and Leon Ehrenpreis. In the context of functional analysis, fundamental solutions are usually developed via the Fredholm alternative and explored in Fredholm theory.

Example edit

Consider the following differential equation Lf = sin(x) with

 

The fundamental solutions can be obtained by solving LF = δ(x), explicitly,

 

Since for the unit step function (also known as the Heaviside function) H we have

 
there is a solution
 
Here C is an arbitrary constant introduced by the integration. For convenience, set C = −1/2.

After integrating   and choosing the new integration constant as zero, one has

 

Motivation edit

Once the fundamental solution is found, it is straightforward to find a solution of the original equation, through convolution of the fundamental solution and the desired right hand side.

Fundamental solutions also play an important role in the numerical solution of partial differential equations by the boundary element method.

Application to the example edit

Consider the operator L and the differential equation mentioned in the example,

 

We can find the solution   of the original equation by convolution (denoted by an asterisk) of the right-hand side   with the fundamental solution  :

 

This shows that some care must be taken when working with functions which do not have enough regularity (e.g. compact support, L1 integrability) since, we know that the desired solution is f(x) = −sin(x), while the above integral diverges for all x. The two expressions for f are, however, equal as distributions.

An example that more clearly works edit

 
where I is the characteristic (indicator) function of the unit interval [0,1]. In that case, it can be verified that the convolution of I with F(x) = |x|/2 is
 
which is a solution, i.e., has second derivative equal to I.

Proof that the convolution is a solution edit

Denote the convolution of functions F and g as Fg. Say we are trying to find the solution of Lf = g(x). We want to prove that Fg is a solution of the previous equation, i.e. we want to prove that L(Fg) = g. When applying the differential operator, L, to the convolution, it is known that

 
provided L has constant coefficients.

If F is the fundamental solution, the right side of the equation reduces to

 

But since the delta function is an identity element for convolution, this is simply g(x). Summing up,

 

Therefore, if F is the fundamental solution, the convolution Fg is one solution of Lf = g(x). This does not mean that it is the only solution. Several solutions for different initial conditions can be found.

Fundamental solutions for some partial differential equations edit

The following can be obtained by means of Fourier transform:

Laplace equation edit

For the Laplace equation,

 
the fundamental solutions in two and three dimensions, respectively, are
 

Screened Poisson equation edit

For the screened Poisson equation,

 
the fundamental solutions are
 
where   is a modified Bessel function of the second kind.

In higher dimensions the fundamental solution of the screened Poisson equation is given by the Bessel potential.

Biharmonic equation edit

For the Biharmonic equation,

 
the biharmonic equation has the fundamental solutions
 

Signal processing edit

In signal processing, the analog of the fundamental solution of a differential equation is called the impulse response of a filter.

See also edit

References edit

  • "Fundamental solution", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
  • For adjustment to Green's function on the boundary see Shijue Wu notes.