The Euler characteristic of any polyhedron homeomorphic to a sphere is 2. For instance, for a cube we have 8  12 + 6 = 2 and for a tetrahedron we have 4  6 + 4 = 2.
In general, the Euler characteristic is a topological invariant, i.e., any two polyhedra that are homeomorphic to each other have the same Euler characteristic. One can therefore extend the definition to more general surfaces than polyhedra, and speak of the Euler characteristic of, for example, a torus, which would be the Euler characteristic of any polyhedron homeomorphic to a torus. In this sense, a torus has Euler characteristic 0.
One can also define the concept of Euler characteristic of manifolds of dimension other than 2. One approach is to define the Euler characteristic of any simplicial complex as the alternating sum
The Euler characteristic χ of a manifold is closely related to its genus g: if the manifold is orientable, we have
More generally still, for any topological space, we can define the nth Betti number[?] b_{n} as the rank of the nth simplicial homology group. The Euler characteristic can then be defined as the alternating sum
The concept of Euler characteristic of a bounded finite partially ordered set is another generalization, important in combinatorics. A poset is "bounded" if it has smallest and largest elements, which let us call 0 and 1. The Euler characteristic of such a poset is μ(0,1), where μ is the Möbius function in that poset's incidence algebra.
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