1. The problem statement, all variables and given/known data I'm trying to understand a proof for Mersenne number theorem, but I'm having trouble understanding the expansion from 2ab-1 into (2a-1)((2a)b-1+(2a)b-2+....+2a+1)). 2. Relevant equations Not sure 3. The attempt at a solution I don't know where to start really and I can't seem to find any proof that explains what is going on in this step.