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Elliptic partial differential equation : ウィキペディア英語版
Elliptic partial differential equation
An elliptic partial differential equation is a general partial differential equation of second order of the form
: Au_ + 2Bu_ + Cu_ + Du_x + Eu_y + F = 0\,
that satisfies the condition
:B^2 - AC < 0.\
(Assuming implicitly that u_=u_. )

Just as one classifies conic sections and quadratic forms based on the discriminant B^2 - 4AC, the same can be done for a second-order PDE at a given point. However, the discriminant in a PDE is given by B^2 - AC, due to the convention (discussion and explanation here). The above form is analogous to the equation for a planar ellipse:
: Ax^2 + 2Bxy + Cy^2 + \cdots = 0 , which becomes (for : u_=u_=0) :
: Au_ + Cu_ + Du_x + Eu_y + F = 0 , and Ax^2 + Cy^2 + \cdots = 0 . This resembles the standard ellipse equation: +-1=0.
In general, if there are ''n'' independent variables ''x''1, ''x''2 , ..., ''x''''n'', a general linear partial differential equation of second order has the form
: L u =\sum_^n\sum_^n a_ \frac \quad \text =0 \,, where L is an elliptic operator.
For example, in three dimensions (x,y,z) :
:a\frac + b\frac + c\frac + d\frac + e\frac \text= 0,
which, for completely separable u (i.e. u(x,y,z)=u(x)u(y)u(z) ) gives
:a\frac + c\frac + e\frac \text= 0.
This can be compared to the equation for an ellipsoid; ++=1.
== See also ==

* Elliptic operator
* Hyperbolic partial differential equation
* Parabolic partial differential equation
*PDEs of second order, for fuller discussion

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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