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Green's function for the three-variable Laplace equation : ウィキペディア英語版
Green's function for the three-variable Laplace equation

In physics, the Green's function (or fundamental solution) for Laplace's equation in three variables is used to describe the response of a particular type of physical system to a point source. In particular, this Green's function arises in systems that can be described by Poisson's equation, a partial differential equation (PDE) of the form
: \nabla^2u(\mathbf) = f(\mathbf)
where \nabla^2 is the Laplace operator in \mathbb^3, f(\mathbf) is the source term of the system, and u(\mathbf) is the solution to the equation. Because \nabla^2 is a linear differential operator, the solution u(\mathbf) to a general system of this type can be written as an integral over a distribution of source given by f(\mathbf):
: u(\mathbf) = \int_ G(\mathbf,\mathbf)f(\mathbf)d\mathbf'
where the Green's function for Laplace's equation in three variables G(\mathbf,\mathbf) describes the response of the system at the point \mathbf to a point source located at \mathbf:
:\nabla^2 G(\mathbf,\mathbf) = \delta(\mathbf-\mathbf)
and the point source is given by \delta(\mathbf-\mathbf), the Dirac delta function.
==Motivation==
One physical system of this type is a charge distribution in electrostatics. In such a system, the electric field is expressed as the negative gradient of the electric potential, and Gauss's law in differential form applies:
:\mathbf = - \mathbf \phi(\mathbf)
:\mathbf \cdot \mathbf = \frac
Combining these expressions gives
:-\mathbf^2 \phi(\mathbf) = \frac (Poisson's equation.)
We can find the solution \phi(\mathbf) to this equation for an arbitrary charge distribution by temporarily considering the distribution created by a point charge q located at \mathbf:
:\rho(\mathbf) = q\delta(\mathbf-\mathbf)
In this case,
:-\frac\mathbf^2\phi(\mathbf) = \delta(\mathbf-\mathbf)
which shows that G(\mathbf, \mathbf) for -\frac\nabla^2 will give the response of the system to the point charge q. Therefore, from the discussion above, if we can find the Green's function of this operator, we can find \phi(\mathbf) to be
: \phi(\mathbf) = \int_ G(\mathbf,\mathbf)\rho(\mathbf)d\mathbf'
for a general charge distribution.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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