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On combinatorial Gauss-Bonnet Theorem for general Euclidean simplicial complexes |
Stephan KLAUS( ) |
| Scientific Administrator of the MFO and Adjunct Professor at Mainz University, Mathematisches Forschungsinstitut Oberwolfach gGmbH (MFO), Schwarzwaldstrasse 9-11, D-77709 Oberwolfach-Walke, Germany |
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Abstract For a finitely triangulated closed surface M2, let αx be the sum of angles at a vertex x. By the well-known combinatorial version of the 2-dimensional Gauss-Bonnet Theorem, it holds ∑x(2π−αx) = 2πχ(M2), where χ denotes the Euler characteristic of M2, αx denotes the sum of angles at the vertex x, and the sum is over all vertices of the triangulation. We give here an elementary proof of a straightforward higher-dimensional generalization to Euclidean simplicial complexes K without assuming any combinatorial manifold condition. First, we recall some facts on simplicial complexes, the Euler characteristics and its local version at a vertex. Then we define δ(τ) as the normed dihedral angle defect around a simplexτ. Our main result is ∑τ (−1)dim(τ)δ(τ) =χ(K), where the sum is over all simplices τ of the triangulation. Then we give a definition of curvature κ(x) at a vertex and we prove the vertex-version ∑x∈K0κ(x) =χ(K) of this result. It also possible to prove Morse-type inequalities. Moreover, we can apply this result to combinatorial (n + 1)-manifolds W with boundary B, where we prove that the difference of Euler characteristics is given by the sum of curvatures over the interior of W plus a contribution from the normal curvature along the boundary B:χ(W) −12χ(B) = ∑τ∈W−B(−1)dim(τ)δ(τ) +∑τ∈B(−1)dim(τ)ρ(τ).
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| Keywords
Curvature
dihedral angle
Euclidean simplex
triangulation
Euler characteristic
Euler manifold
combinatorial manifold
pseudo manifold
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Corresponding Author(s):
Stephan KLAUS
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Issue Date: 23 September 2016
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