Euler's relation and convexity
You can get persuaded of the the fact that convexity is unimportant for the validity of Euler's relation (Vertices - Edges + Faces = 2) by imagining to "enlarge" this polyhedron in its "belt". This operation doesn't change the number of vertices, edges and faces, but the concave polyhedron becomes convex.
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Euler relation (3D geometry)
