1. The problem statement, all variables and given/known data Q: If A and B are both nxn matrices and AB-I is invertable then prove that BA-I is also invertable. 2. Relevant equations if A is invertible iff |A|<>0 3. The attempt at a solution I've been thinking about this for over an hour I've only managed to prove it if either A or B are invertable. because if let's say A is invertable then: |AB-I|<>0 => |AB-I||A|<>0 => |ABA-A|<>0 => |A||BA-I|<>0 => |BA-I|<>0 and so it's invertable. if B is invertable then you do pretty much the same thing on starting on the left side. But what if they're both singular? Thanks.