# Summation notation indexing

1. Jan 28, 2010

### Somefantastik

$$\sum_{k=0}^{\infty}a_{k}(k+r)(k+r-1)x^{k-1}$$

I need to get my x term to look like xn.

If I set n = k-1, then that makes my index start at n = -1, which is silly. What can I do?

2. Jan 28, 2010

### rochfor1

Why is that silly? When k = 0, you have x^-1 in your original summation.

3. Jan 29, 2010

### Somefantastik

It doesn't make sense when I'm using it in the Frobenius method.

Last edited: Jan 29, 2010