A square matrix P is called an idempotent if P^2 = P,
Show that if A is n x m and B is m x n, and if AB = In, then BA is an idempotent
The Attempt at a Solution
I have this one identity I think, but I'm not sure how to use it?
If A is m x n, then ImA = A = AIn
can I do anything with that?
My inverse definition shows for square matrices... how does it apply for non-square matrices? Or how can I use it?