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 "squeeze" this polyhedron in its "belt". This operation doesn't change the number of vertices, edges and faces, but the convex polyhedron becomes concave.
© matematita
The image belongs to the sections...:
Euler relation (3D geometry)
