Ais an[img=44x19]1803b6a4c8dddc0.png[/img]matrix. A is not invertible if and only if 0 is an eigenvalue of A.

Let A be a 4 by 3 matrix, and r(A)=2, and B is an invertible matrix, then r(AB)=______

If I-AB is invertible, then I-BA is invertible. Where I is the identity matrix, and A is m by n, B is n by m.