Homology and Integration; Req. for Proof.

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Discussion Overview

The discussion revolves around the relationship between homotopy of curves in a domain of the complex plane and the integrals of continuously-differentiable functions along these curves. Participants explore the conditions under which the integrals of such functions remain invariant under homotopy, referencing concepts from homology and theorems like Green's theorem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant queries the proof that if two curves C and C' in a domain D are homotopic, then the integrals of a continuously-differentiable function f over these curves are equal.
  • Another participant seeks clarification on the meaning of C^1(D), later specifying that it refers to functions that are continuously-differentiable in D.
  • There is a suggestion that the result may be a corollary of Green's theorem, although this is not confirmed.
  • A participant provides an example of integrating a specific function along the edges of a unit square, noting the results of the integrals along different segments.
  • One participant reflects on the definition of homology of simple-closed curves as described in Rotman's Homological Algebra, expressing uncertainty about the conditions on the function f.
  • Another participant states that if a 1-form is closed, then its integral on homotopic curves will be the same, linking this to the Cauchy-Riemann equations and the concept of holomorphic functions.
  • There is mention of Stokes' theorem in the plane and its classical relation to Green's theorem, although the applicability to the current context is not fully resolved.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the specific conditions required for the integrals to be equal under homotopy, and there is no consensus on the applicability of Green's theorem or the definitions involved in homology.

Contextual Notes

Limitations include unclear assumptions about the function f, particularly whether it is complex analytic, and the specific conditions under which the integrals are considered equal in homology.

Bacle
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Hi, All:
Let D be a domain (open+ connected) in the complex plane.

I was wondering if someone had a ref. for the proof that if two
curves C,C' in D are homotopic, thenfor f in C^1(D), Int_C F= Int_C' f, and/or if this
theorem has a standard name.

Thanks.
 
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Bacle said:
Hi, All:
Let D be a domain (open+ connected) in the complex plane.

I was wondering if someone had a ref. for the proof that if two
curves C,C' in D are homotopic, thenfor f in C^1(D), Int_C F= Int_C' f, and/or if this
theorem has a standard name.

Thanks.

What is C^1(D)?
 
I meant f is continuously-differentiable in D. This seems like it may be
a corollary of Green's thm., but I am not sure.
 
Bacle said:
I meant f is continuously-differentiable in D. This seems like it may be
a corollary of Green's thm., but I am not sure.

Integrate
f(x,y) = xy^{2}

along the edges of the unit square.

Along the edges (0,0) to (1,0) and (0,0) to (0,1) the integral is zero.
Along the edge (1,0) to (1,1) the integral is 1/3.
From (0,1) to (1,1) the integral is 1.

Did you mean the integral of df?
 
You're right, I think I do not have the conditions right. I was thinking of a def. of

homology of SCCurves, as defined in Rotman's Homological Algebra book

( I don't have the book with me at the moment; not in my school's library):

We are working in a subset S of R^2 , and f is defined there ; I am

not sure if f is assumed (complex) analytic,but then two simple-closed curves C,C' in S are homologous

if (Def.) Int_C f= Int_C' f , so that Int_(C-C') f = Int_0 f , so I cannot remember

the actual conditions on f. But basically the result is that the integral of f is constant

in homology.

Still, any chance you have a ref. for Int dz ?
 
Last edited:
Bacle said:
You're right, I think I do not have the conditions right. I was thinking of a def. of

homology of SCCurves, as defined in Rotman's Homological Algebra book

( I don't have the book with me at the moment; not in my school's library):

We are working in a subset S of R^2 , and f is defined there ; I am

not sure if f is assumed (complex) analytic,but then two simple-closed curves C,C' in S are homologous

if (Def.) Int_C f= Int_C' f , so that Int_(C-C') f = Int_0 f , so I cannot remember

the actual conditions on f. But basically the result is that the integral of f is constant

in homology.

Still, any chance you have a ref. for Int dz ?

The theorem is if a 1-form is closed then its integral on homotopic curves will be the same. If f is holomorphic in the domain then the 1 form fdz is closed. This follows from the Cauchy-Riemann equations. (For complex 1-forms closed means that the real and imaginary parts are both closed.)

The general theorem is Stokes theorem in the plane which classically I think is Green's theorem.
 

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