# Thread: Independence of Path

1. ## Independence of Path

i am a little confused by the idea of "independence of path" in complex analysis. i understand that certain functions, when integrated over simple paths with the same endpoints a and b yield the same value. however i do not understand how to determine whether the function is independent of the path (without calculating over arbitrary simple paths).

can anyone help enlighten me? thanks in advance!

2. A fundamental theorem due to the French mathematician Augustin Cauchy extablishes that if $\displaystyle f(z)$ is analytic in a region inside a closed path $\displaystyle \gamma$ is...

$\displaystyle \displaystyle \int_{\gamma} f(z)\ dz =0$ (1)

Now if $\displaystyle f(z)$ is analytic in the whole complex plane [i.e. is an entire function...] the (1) is valid independently from $\displaystyle \gamma$ , and that means that, if we devide $\displaystyle \gamma$ in two parts, one coming from $\displaystyle z_{1}$ to $\displaystyle z_{2}$ and the other coming back from $\displaystyle z_{2}$ to $\displaystyle z_{1}$ is...

$\displaystyle \displaystyle \int_{z_{1}}^{z_{2}} f(z)\ dz + \int_{z_{2}}^{z_{1}} f(z)\ dz =0$ (2)

... and that means that $\displaystyle \displaystyle \int_{z_{1}}^{z_{2}} f(z)\ dz$ depends only from $\displaystyle z_{1}$ and $\displaystyle z_{2}$ and doesn't depend from the path connecting $\displaystyle z_{1}$ and $\displaystyle z_{2}$...

Kind regards

$\displaystyle \chi$ $\displaystyle \sigma$

3. Originally Posted by chisigma
A fundamental theorem due to the French mathematician Augustin Cauchy extablishes that if $\displaystyle f(z)$ is analytic in a region inside a closed path $\displaystyle \gamma$ is...

$\displaystyle \displaystyle \int_{\gamma} f(z)\ dz =0$ (1)

Now if $\displaystyle f(z)$ is analytic in the whole complex plane [i.e. is an entire function...] the (1) is valid independently from $\displaystyle \gamma$ , and that means that, if we devide $\displaystyle \gamma$ in two parts, one coming from $\displaystyle z_{1}$ to $\displaystyle z_{2}$ and the other coming back from $\displaystyle z_{2}$ to $\displaystyle z_{1}$ is...

$\displaystyle \displaystyle \int_{z_{1}}^{z_{2}} f(z)\ dz + \int_{z_{2}}^{z_{1}} f(z)\ dz =0$ (2)

... and that means that $\displaystyle \displaystyle \int_{z_{1}}^{z_{2}} f(z)\ dz$ depends only from $\displaystyle z_{1}$ and $\displaystyle z_{2}$ and doesn't depend from the path connecting $\displaystyle z_{1}$ and $\displaystyle z_{2}$...

Kind regards

$\displaystyle \chi$ $\displaystyle \sigma$
It doesn't need to be entire. Only holomorphic on a path connected subspace.

4. Originally Posted by Drexel28
It doesn't need to be entire. Only holomorphic on a path connected subspace.
In order to avoid misunderstanding a simple example. Susppose that is $\displaystyle f(z)=\frac{1}{z}$, $\displaystyle z_{1}= -i\ \sqrt{2}$, $\displaystyle z_{2}= i\ \sqrt{2}$ and $\displaystyle \gamma$ is the 'red circle' in the figure...

By the residue theorem is...

$\displaystyle \displaystyle \int_{\gamma} \frac{dz}{z} = 2\ \pi\ i$ (1)

... and that means that $\displaystyle \displaystyle \int_{z_{1}}^{z_{2}} \frac{dz}{z}$ will be $\displaystyle \pi i$ if the point $\displaystyle z=0$ is on the left of the half circle connecting $\displaystyle z_{1}$ to $\displaystyle z_{2}$ and $\displaystyle - \pi\ i$ if it is on the right...

Kind regards

$\displaystyle \chi$ $\displaystyle \sigma$

5. I think that Drexel is partially correct. You don't need the function to be entire. However, it must be defined on an open simply-connected domain.

See Cauchy's integral theorem - Wikipedia, the free encyclopedia.

6. Originally Posted by roninpro
I think that Drexel is partially correct. You don't need the function to be entire. However, it must be defined on an open simply-connected domain.

See Cauchy's integral theorem - Wikipedia, the free encyclopedia.
Ah crap. I always say the wrong hypothesis. You'd think someone who's interested in alg. top. I'd remember haha