# Is there a name for infinite series of this type?

I asked a question about this series;

$(1 - \frac12)+(\frac13 - \frac14)(1 - \frac12 + \frac13)+(\frac15 - \frac16)(1 - \frac12 + \frac13 - \frac14 + \frac15)+(\frac17 - \frac18)(1 - \frac12 + \frac13 - \frac14 + \frac15 - \frac16 + \frac17)+...$

in a previous thread and something else about it that I'd like to know is if there is a name for series where the coefficient of each term is a partial sum? Furthermore, is there a general method for finding the closed form sums of such series?

## 1 Answer

There is no special name since what you have is just a double summation instead of a single summation. Your series is nothing but $$\sum_{n=0}^{\infty} \sum_{k=1}^{2n+1} \left(\dfrac1{2n+1}-\dfrac1{2n+2}\right) \left(\dfrac{(-1)^{k-1}}{k} \right)$$ which can also be written as a single summation $$\sum_{n=0}^{\infty} \left(\dfrac1{2n+1}-\dfrac1{2n+2}\right) \left(H_{2n+1} - H_n\right)$$

• Ah so it is a double sum. Thought so. Sorry to bother you with such a trivial question it was just that sometimes it's written with the sigma in the middle which somehow confused me! – KingChem Dec 29 '12 at 0:57