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单词 CauchyKowalewskiTheorem
释义

Cauchy-Kowalewski theorem


Consider a system of partial differential equationsMathworldPlanetmath involving m dependent variables u1,,um and n+1 independent variables t,x1,,xn:

uit=Fi(u1,,um;t,x1,,xn;u1x1,,umxn)

in which F1,,Fm are analytic functions in a neighborhood of a point (u10,,um0;t0,x10,,xn0) subject to the boundaryconditionsMathworldPlanetmath

ui=fi(x1,,xn)

when t=t0 for given functionsMathworldPlanetmath fi which are analytic in a neighborhood of x10,,xn0 such that ui0=fi(x10,,xn0).

The Cauchy-Kowalewski theorem asserts that this boundary value problem has a unique analytic solution ui=fi(t,x1,,xn) in a neighborhood of (u10,,um0;0,x10,,xn0).

The Cauchy-Kowalewski theorem is a local existence theoremMathworldPlanetmath — it only asserts that a solution exists in a neighborhood of the point, not in all space. A peculiar feature of this theorem is that the type of the differential equation (whether it is elliptic, parabolic, or hyperbolic) is irrelevant. As soon as one either considers global solutions or relaxes the assumptionPlanetmathPlanetmath of analyticity, this is no longer the case — the existence and uniqueness of solutions to a differential equation (or system of differential equations) will depend upon the type of the equation.

By simple transformations, one can generalize this theorem.

By making a change of variable t=t-ϕ(x1,,xm), with ϕ analytic, one can generalize the theorem to the case where the boundary values are specified on a surface given by the equation t=ϕ(x1,,xm) rather than on the plane t=t0.

One can allow higher order derivatives by the device of introducing new variables. For instance, to allow third order time derivatives of u1, one could introduce new variables u1t and u1tt and augment the system of equations by adding

u1t=u1t

and

u1tt=u1tt

Likewise, one can eliminate spatial derivativesPlanetmathPlanetmath. The manner in which one introduces new equations for these new variables is somewhat clumsy to describe in general, and it is best to explain it by example, as is done in a http://planetmath.org/node/6198supplement to this entry.

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更新时间:2025/5/4 10:02:21