Take any tetrahedron, and for each face compute its perimeter P, and the solid angle Ω it subtends as seen from the opposite vertex.
Daniil Rudenko and @robinhouston have found a linear relationship between 1/P and cot(Ω/2).
https://mathoverflow.net/questions/336464/a-curious-relation-between-angles-and-lengths-of-edges-of-a-tetrahedron/336777 …pic.twitter.com/6a8q1uzCrX
That’s amazing.. it makes me wonder if it’s a 3D generalization of the famous V-E+F=2 relation ..
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TL;DR: the alternating sum of numbers of k-dim 'cells' (points - edges + faces - solid 3D cells + ...) is a topological invariant χ.
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The Euler characteristic is very cool, and has lots of nice properties and generalisations: https://en.wikipedia.org/wiki/Euler_characteristic … But I don’t think this result is connected to it.
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