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单词 ProofOfGeneralStokesTheorem
释义

proof of general Stokes theorem


We divide the proof in several steps.

Step One.

Suppose M=(0,1]×(0,1)n-1 and

ω(x1,,xn)=f(x1,,xn)dx1dxj^dxn

(i.e. the term dxj is missing).Hence we have

dω(x1,,xn)=(fx1dx1++fxndxn)dx1dxj^dxn
=(-1)j-1fxjdx1dxn

and from the definition of integral on a manifold we get

M𝑑ω=0101(-1)j-1fxj𝑑x1𝑑xn.

From the fundamental theorem of CalculusMathworldPlanetmathPlanetmath we get

M𝑑ω=(-1)j-10101^01f(x1,,1,,xn)-f(x1,,0,,xn)dx1dxj^dxn.

Since ω and hence f have compact support in M we obtain

M𝑑ω={0101f(1,x2,,xn)𝑑x2𝑑xnifj=10ifj>1.

On the other hand we notice thatMωis to be understood as Mi*ω wherei:MM is the inclusion mapMathworldPlanetmath.Hence it is trivial to verify that when j1 then i*ω=0while if j=1 it holds

i*ω(x)=f(1,x2,,xn)dx2dxn

and hence, as wanted

Mi*ω=0101f(1,x2,,xn)𝑑x2𝑑xn.

Step Two.

Suppose now that M=(0,1]×(0,1)n-1 and let ω be any differential formMathworldPlanetmath.We can always write

ω(x)=jfj(x)dx1dxj^dxn

and by the additivity of the integral we can reduce ourself to the previous case.

Step Three.

When M=(0,1)n we could follow the proof as in the first caseand end up withM𝑑ω=0while, in fact, M=.

Step Four.

Consider now the general case.

First of all we consider an oriented atlas (Ui,ϕi) such that eitherUi is the cube(0,1]×(0,1)n-1 or Ui=(0,1)n.This is always possible. In fact given any open set U in [0,+)×n-1 and a point xU up to translations and rescaling it is possibleto find a “cubic” neighbourhood of x contained in U.

Then consider a partition of unity αi for this atlas.

From the properties of the integral on manifolds we have

M𝑑ω=iUiαiϕ*𝑑ω=iUiαid(ϕ*ω)
=iUid(αiϕ*ω)-iUi(dαi)(ϕ*ω).

The second integral in the last equality is zero sinceidαi=diαi=0, while applying the previous steps to the first integral we have

M𝑑ω=iUiαiϕ*ω.

On the other hand, being(Ui,ϕ|Ui) an oriented atlas for M and being αi|Ui a partition of unity, we have

Mω=iUiαiϕ*ω

and the theorem is proved.

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更新时间:2025/5/4 7:06:13