Math Help - Power Series

1. Power Series

is there a closed-form for the following power series?
$\sum_{i = a}^{\infty} i \cdot x^{n \cdot i}$

p.s. i just know that
1. $\sum_{i = 0}^{\infty} x^{n \cdot i} = \frac{1}{1- x^n}$
2. $\sum_{i = 0}^{\infty} i \cdot x^{i} = \frac{x}{(1-x)^2}$

Thanks

2. Originally Posted by graticcio
is there a closed-form for the following power series?
$\sum_{i = a}^{\infty} i \cdot x^{n \cdot i}$

p.s. i just know that
1. $\sum_{i = 0}^{\infty} x^{n \cdot i} = \frac{1}{1- x^n}$
2. $\sum_{i = 0}^{\infty} i \cdot x^{i} = \frac{x}{(1-x)^2}$

Thanks
Yes. Replace x with x^n in formula 2.