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biharmonic equation : ウィキペディア英語版
biharmonic equation
In mathematics, the biharmonic equation is a fourth-order partial differential equation which arises in areas of continuum mechanics, including linear elasticity theory and the solution of Stokes flows. It is written as
:\nabla^4\varphi=0
or
:\nabla^2\nabla^2\varphi=0
or
:\Delta^2\varphi=0
where \nabla^4 is the fourth power of the del operator and the square of the laplacian operator \nabla^2 (or \Delta), and it is known as the biharmonic operator or the bilaplacian operator. In summation notation, it can be written in n dimensions as:
:
\nabla^4\varphi=\sum_^n\sum_^n\partial_i\partial_i\partial_j\partial_j \varphi.

For example, in three dimensional cartesian coordinates the biharmonic equation has the form
:
+
+
+
2+
2+
2 = 0.

As another example, in ''n''-dimensional Euclidean space,
:\nabla^4 \left(\right)=
where
:r=\sqrt.
which, for ''n=3 and n=5'' only, becomes the biharmonic equation.
A solution to the biharmonic equation is called a biharmonic function. Any harmonic function is biharmonic, but the converse is not always true.
In two-dimensional polar coordinates, the biharmonic equation is
:
\frac \frac \left(r \frac \left(\frac \frac \left(r \frac\right)\right)\right)
+ \frac \frac
+ \frac \frac
- \frac \frac
+ \frac \frac = 0

which can be solved by separation of variables. The result is the Michell solution.
==2-dimensional space==
The general solution to the 2-dimensional case is
:
x v(x,y) - y u(x,y) + w(x,y)

where u(x,y), v(x,y) and w(x,y) are harmonic functions and v(x,y) is a harmonic conjugate of u(x,y).
Just as harmonic functions in 2 variables are closely related to complex analytic functions, so are biharmonic functions in 2 variables. The general form of a biharmonic function in 2 variables can also be written as
:
\operatorname(\barf(z) + g(z))

where f(z) and g(z) are analytic functions.

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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