# Is this acceptable in recursive formulas?

1. Jan 19, 2012

### Cloudless

Say for the recursive formula 2, 5, 9, 19 , 37...

Instead of writing it as an = an-1 + * 2 an-2

can you write it as:

an = 2an + (-1)n

Just double checking. I encountered this earlier helping someone and I haven't done recursive formulas in a while..

Last edited: Jan 19, 2012
2. Jan 19, 2012

### AlephZero

I think you meant an = 2an-1 + (-1)n

Sure. Your formula is the same as

an = an-1 + (-1)n + an-1
= an-1 + (-1)n + 2an-2 + (-1)n-1
= an-1 + 2an-2