1. The problem statement, all variables and given/known data Prove in general that if m does not equal n, then AB and BA cannot both be identity matrices, where A is mxn and B is nxm. 2. Relevant equations None (that I know of at least). 3. The attempt at a solution At first I thought it would be a good idea to define each element in A and B and write out some elements from AB and BA, and hope that I noticed a pattern where I would see something possible only if n=m. This proved very cumbersome and I could not get it to go anywhere. Next I tried assuming that both AB and BA equaled identity matrices of appropriate dimensions, with the intention of deriving a contradiction, but I was unfortunately unavle to do so. I appreciate any help, as I really don't know what to try next.