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I am have troubles with the following proof of the global Gauss-Bonnet which take the form;

Let $M$ be a compact regular surface in $\mathbb{R}^3$. If $K$ is the Gaussian curvature of $M$ then

$\int_{M} KdA=2 \pi \chi(M)$,

where $\chi$ is the Euler characeristic.

The proof is presented as followsenter image description here

(Source: "An Introduction to Gaussian Geometry" by Sigmundur Gudmundsson, http://www.matematik.lu.se/matematiklu/personal/sigma/Gauss.pdf.)

Theorem $8.5$ is simply a local Gauss-Bonnet theorem.

Now, I can't figure out how the number of edges $E$ and vertices $V$ are obtained in the last equation, could someone help me out with this?

Nate Eldredge
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Number4
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1 Answers1

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As indicated in the comments, $n_k$ is the number of corners of the $k^{\text{th}}$ polygon, and $\alpha_{ki}$ is the angle of that polygon at its $i^{\text{th}}$ corner.

Perhaps rewriting the last three equations a bit differently might help: \begin{align*} \int_M K \, dA &= \sum_{k=1}^F \bigl( (2-n_k) \pi + \sum_{i=1}^{n_k} \alpha_{ki} \bigr) \\ &= 2 \pi F - 2 \pi \cdot \frac{1}{2} \sum_{k=1}^F n_k + 2 \pi \cdot \frac{1}{2\pi} \sum_{k=1}^F \sum_{i=1}^{n_k} \alpha_{ki} \end{align*}

We have $\frac{1}{2} \sum_{k=1}^F n_k = E$, equivalently $\sum_{k=1}^F n_k = 2E$, because the set of edges is obtained from the set of sides of the polygons by identifying those sides in pairs.

We have $\frac{1}{2\pi} \sum_{k=1}^F \sum_{i=1}^{n_k} \alpha_{ki} = V$ because the sum can first be rewritten as a sum over the vertex set of the sum of all the angles meeting at each vertex, but each of the latter sums equals $2\pi$.

Lee Mosher
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  • isnt all $n_{k}=3$, as each is a triangle? Moreover in that case suppose we have $1$ triangle, then $3=2$. Or, which is probably the case, am I misunderstanding this? – Number4 Aug 06 '19 at 10:10
  • According to the wording of that copied text, yes each is a triangle, and so yes we should have each $n_k=3$; I suppose this is just an editorial error in that text. I don't understand the rest of your comment, although I'll add that one triangle is not possible, in fact an odd number of triangles is not possible, because an odd number of edges cannot be identified in pairs. – Lee Mosher Aug 06 '19 at 13:06
  • you are saying that the proof implies that a "triangulation" is done with an even number of triangles necessarily? and we can always archive this simple by splitting up the last one? – Number4 Aug 06 '19 at 13:18
  • When you glue up a bunch of polygons to form a surface by identifying sides in pairs, their must clearly be an even number of sides, else the sides cannot be grouped into pairs. If the bunch of polygons consists of $n$ triangles, then the number of sides that must be paired up is $3n$. What happens if $n$ is odd? – Lee Mosher Aug 06 '19 at 13:24
  • Given this scheme, how are the triangles allowed to beeing joined? – Number4 Aug 06 '19 at 14:32
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    @Lobsided: It seems that you might profit from reading about how surfaces are constructed by gluing polygons. Trying to give you a course on this topic in the comments to an answer to your question is not good practice on this site. I suggest that you look up topics on the "Classification of Surfaces"; there seem to be several good links that come up if you search on that term. Massey's book also covers this topic in detail. – Lee Mosher Aug 06 '19 at 14:42
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    As a very brief answer, one you have paired up the sides of the set of triangles, for each side pair $A,B$ one chooses a homeomorphism $h : A \to B$ and one identifies each $x \in A$ to $h(x) \in B$. – Lee Mosher Aug 06 '19 at 14:43
  • I been reading about this. And it got me confused. According to wikipedia we have the geodesic curvature integral in the equality of the theorem. But if every compact surface have a geodesic triangulation then this term would never appear for any version of thr theorem with or without boundary. Are you sure that geodesic triangulation is possible? – Number4 Aug 12 '19 at 10:45
  • This might be a good question, but it's bad practice on math.stackexchange to bury a new question in a comment to an answer to an old question, where only the answerer will ever see it. Perhaps you can formulate a new question to post about this. – Lee Mosher Aug 12 '19 at 14:27
  • https://math.stackexchange.com/questions/3321847/question-on-the-gauss-bonnet-theorem – Number4 Aug 13 '19 at 08:20