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## Homework Statement

I've been given the series 3,4,6,10,18... and asked to express as the ∑a

_{r}, between r=0 and r = infinity.

## Homework Equations

## 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'.