  The Euler characteristic of a cell complex is often thought of as the
alternating sum of the number of cells of each dimension. When the complex is
infinite, the sum diverges. Nevertheless, it can sometimes be evaluated; in
particular, this is possible when the complex is the nerve of a finite
category. This provides an alternative definition of the Euler characteristic
of a category, which is in many cases equivalent to the original one
(math.CT/0610260).
