Re: Name of concept?

Incidentally, did you know that if Z and Z' are defined so that a d a' 0 --> Z ---> C ---> C ---> Z' ---> 0 is exact, then the homology is the image (= coimage) of a'.a: Z --> Z'? This is a triviality, but it gives a symmetric definition of homology. Notice that it defines something even when d.d is not 0. I guess it is Z mod Z meet ker(d). Mike