Euler's polyhedron theorem states for a polyhedron
where
As is well known, Euler's formula is not true for all polyhedra. The condition on polyhedra considered here is that of being a homology sphere, which says that the cycles (chains whose boundary is zero) are exactly the bounding chains (chains that are the boundary of a chain of one higher dimension).
The present proof actually goes beyond the three-dimensional version of the polyhedral formula given by Lakatos; it is dimension-free, in the sense that it gives a formula in which the dimension of the polyhedron is a parameter. The classical Euler relation
The main theorem, expressed in the language of the present article, is
Sum alternating - characteristic - sequence (
where
where
Two other special cases are proved: the first says that a one-dimensional "polyhedron" that is a homology sphere consists of just two vertices (and thus consists of just a single edge); the second special case asserts that a two-dimensional polyhedron that is a homology sphere (a polygon) has as many vertices as edges.
A treatment of the more general version of Euler's relation can be found in [12] and [6]. The former contains a proof of Steinitz's theorem, which shows that the abstract polyhedra treated in Poincaré's proof, which might not appear to be about polyhedra in the usual sense of the word, are in fact embeddable in
MML identifier: POLYFORM, version: 7.8.05 4.89.993