Encyclosphere.org ENCYCLOREADER
  supported by EncyclosphereKSF

Elliptic partial differential equation

From Encyclopedia of Mathematics - Reading time: 1 min


at a given point $ x $

A partial differential equation of order $ m $,

$$ \sum a _ {i _ {1} \dots i _ {n} } ( x) \frac{\partial ^ {m} u }{\partial x _ {1} ^ {i _ {1} } \dots \partial x _ {r} ^ {i _ {n} } } + L _ {1} u = f ,\ \ \sum_{j=1}^ { n } i _ {j} = m , $$

such that $ L _ {1} $ is a differential operator of order less than $ m $, whose characteristic equation at $ x $,

$$ K ( \lambda _ {1} \dots \lambda _ {n} ) = \sum a _ {i _ {1} \dots i _ {n} } ( x) \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } = 0 ,\ \sum _ { j=1}^ { n } i _ {j} = m , $$

has no real roots except $ \lambda _ {1} = 0 \dots \lambda _ {n} = 0 $.

For second-order equations the characteristic form is quadratic,

$$ Q ( \lambda _ {1} \dots \lambda _ {n} ) = \sum _ {i , j = 1 } ^ { n } A _ {ij} ( x) \lambda _ {i} \lambda _ {j} , $$

and can be brought to the form

$$ Q = \sum_{i=1}^ { n } \alpha _ {i} \xi _ {i} ^ {2} $$

by a non-singular affine transformation of the variables $ \lambda _ {i} = \lambda _ {i} ( \xi _ {1} \dots \xi _ {n} ) $, $ i = 1 \dots n $.

When all $ \alpha _ {i} = 1 $ or all $ \alpha _ {i} = - 1 $, the equation is said to be of elliptic type.

A partial differential equation is said to be of elliptic type in its domain of definition if it is elliptic at every point of this domain.

An elliptic partial differential is called uniformly elliptic if there are positive numbers $ k _ {0} $ and $ k _ {1} $ such that

$$ k _ {0} \sum_{i=1}^ { n } \lambda _ {i} ^ {2} \leq Q ( \lambda _ {1} \dots \lambda _ {n} ) \leq k _ {1} \sum_{i=1}^ { n } \lambda _ {i} ^ {2} . $$

For references see Differential equation, partial.


References[edit]

[a1] L.V. Hörmander, "The analysis of linear partial differential operators" , 1 , Springer (1983) MR0717035 MR0705278 Zbl 0521.35002 Zbl 0521.35001

How to Cite This Entry: Elliptic partial differential equation (Encyclopedia of Mathematics) | Licensed under CC BY-SA 3.0. Source: https://encyclopediaofmath.org/wiki/Elliptic_partial_differential_equation
1 | Status: cached on November 13 2024 00:00:26
↧ Download this article as ZWI file
Encyclosphere.org EncycloReader is supported by the EncyclosphereKSF