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・ Cauchy formula for repeated integration
・ Cauchy horizon
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Cauchy surface
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Cauchy surface : ウィキペディア英語版
Cauchy surface
Intuitively, a Cauchy surface is a plane in space-time which is like an ''instant'' of time; its significance is that giving the initial conditions on this plane determines the future (and the past) uniquely.
More precisely, a Cauchy surface is any subset of space-time which is intersected by every inextensible, non-spacelike (i.e. causal) curve exactly once.
A partial Cauchy surface is a hypersurface which is intersected by any causal curve ''at most'' once.
It is named for French mathematician Augustin Louis Cauchy.
== Discussion ==

If \mathcal is a space-like surface (i.e., a collection of points such that every pair is space-like separated), then D^(\mathcal) is the future of \mathcal, i. e.:
D^+(\mathcal):=\p\text\mathcal)
Similarly D^(\mathcal), the past of \mathcal, is the same thing going forward in time.
When there are no closed timelike curves, D^ and D^ are two different regions. When the time dimension closes up on itself everywhere so that it makes a circle, the future and the past of \mathcal are the same and both include \mathcal. The Cauchy surface is defined rigorously in terms of intersections with inextensible curves in order to deal with this case of circular time. An inextensible curve is a curve with no ends: either it goes on forever, remaining timelike or null, or it closes in on itself to make a circle, a closed non-spacelike curve.
When there are closed timelike curves, or even when there are closed non-spacelike curves, a Cauchy surface still determines the future, but the future includes the surface itself. This means that the initial conditions obey a constraint, and the Cauchy surface is not of the same character as when the future and the past are disjoint.
If there are no closed timelike curves, then given \mathcal a partial Cauchy surface and if D^(\mathcal)\cup \mathcal\cup D^(\mathcal) = \mathcal, the entire manifold, then \mathcal is a Cauchy surface. Any surface of constant t in Minkowski space-time is a Cauchy surface.

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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