How can I adjust the index in summation notation for the Frobenius method?

[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?

 

[rochfor1]

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

 

[Somefantastik]

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