1. The problem statement, all variables and given/known data I've been given the series 3,4,6,10,18... and asked to express as the ∑ar, between r=0 and r = infinity. 2. Relevant equations 3. The attempt at a solution Well, I can see a pattern! The difference between terms doubles every time. I'm having difficulty expressing this mathematically though... I think it should be 3 plus some function of r. Is there a technique for this? All we've been told is to do it 'by inspection'.