Euler characteristic of a topological space X

Euler characteristic of a topological space X

[′ȯi·lər ‚kar·ik·tə′ris·tik əv ə ‚täp·ə¦läj·i·kəl ¦spās ′eks]
(mathematics)
The number χ(X) = Σ(-1) q βq , where βq is the q th Betti number of X.