Can we define the Euler characteristic of an infinite-dimensional CW complex

algebraic-topologycw-complexes

For a finite CW complex $X$, the Euler characteristic $\chi(X)$ is defined to be the alternating sum $\sum_k(-1)^kc_k$ where $c_k$ is the number of $k$-cells of $X$. Note, the Euler characteristic of $X$ is independent of the choice of $CW$ structure on $X$. Then for $H_k(X)$ the $k$-th singular homology group of $X$:
$$ \chi(X)=\sum_k(-1)^k\text{rank }H_k(X).$$

Can we define the Euler characteristic of infinite-dimensional CW complexes in a similar fashion?

Best Answer

Not really. If you want to use the definition as the alternating sum of the number of cells, then it certainly won't work; the sum $\sum_k (-1)^k c_k$ will never converge if an infinite number of $c_k$ are nonzero. If $\operatorname{rank} H_k(X) = 0$ for $k \ge N$ (with a fixed $N$) then you can use the second definition. But if an infinite number of $H_k(X)$ have positive rank, then it won't work.

What can be done is defining the Poincaré polynomial of $X$. It's given by: $$P_X(t) = \sum_{k \ge 0} (\operatorname{rank} H_k(X)) t^k.$$ In general it's an infinite series. If the Euler characteristic is well-defined (i.e. if only a finite number of homology groups have positive ranks), then $\chi(X) = P_X(-1)$. Otherwise, you cannot say much more.