Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

If $S$ is a surface which is the complement of finitely many points in a compact surface, and the metric in $S$ is complete, then is Gauss-Bonnet theorem still valid for $S$?

share|improve this question
add comment

3 Answers

up vote 1 down vote accepted

Sort of.

http://www.ams.org/proc/1982-086-01/S0002-9939-1982-0663893-8/S0002-9939-1982-0663893-8.pdf

share|improve this answer
    
Perfect, thanks! –  hao Oct 24 '10 at 15:48
add comment

No.

If you remove a finite set of points from a compact surface, it will no longer be complete. If you change the metric so that you do have a complete surface, you cannot apply the Gauss Bonnet theorem anyway, because the surface isn't compact.

share|improve this answer
    
You can't apply the Gauss-Bonnet theorem, but its conclusion could still hold. –  Nate Eldredge Oct 24 '10 at 22:35
    
@Nate Eldredge Yes, as the nice two page paper that Max linked to shows. I was clearly too quick to blow the question off. –  yasmar Oct 24 '10 at 23:15
add comment

No.

Take the sphere $S^2$ and remove one point, giving you a surface $S$ diffeomorphic to $\mathbb{R}^2$. So we can equip $S$ with a metric making it isometric to $\mathbb{R}^2$, which is certainly complete. $\mathbb{R}^2$ has zero curvature but its Euler characteristic is $1$, so $$ 0 = \int_{\mathbb{R}^2} K dVol \ne \chi(\mathbb{R}^2) = 1.$$

share|improve this answer
    
Yes, I think I should add one more condition, $S$ has finite area, then it will be true, as shown by Max –  hao Oct 24 '10 at 15:47
    
@hao: What is an example of a complete, connected, noncompact Riemannian manifold of finite area? I'm having trouble thinking of one. –  Nate Eldredge Oct 24 '10 at 22:38
    
If a small disk in the sphere is replaced by a 'spike' extending indefinitely we can get such a surface. I.e., something like half of the Tractricoid, which has finite area. This is the image invoked by the short paper that Max linked to. –  yasmar Oct 24 '10 at 23:24
    
@yasmar: Thanks. I was trying to think of something like that, but I kept coming up with en.wikipedia.org/wiki/Gabriel%27s_Horn instead. –  Nate Eldredge Oct 25 '10 at 16:48
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.