# I was wondering why this works and if it works every time

1. Feb 12, 2008

### end3r7

1. The problem statement, all variables and given/known data
I have to find for which values of x the following converge

2. Relevant equations
$$\sum n x^{n}$$

$$\sum \frac{x^{n}}{n}$$

$$\sum n^{n} x^{x}$$

3. The attempt at a solution

I used the ratio test for the first two and the root test for the last and found respectively that x must lie within

(-1,1)
[-1,1)
{0}

Are these right?
and
Is this the right way to go all the time?

I was wondering when I'll be able to apply the ratio/root tests for radii of convergence. Is it just when we have power functions?

2. Feb 12, 2008

### Dick

For the last one you mean sum(n^n*x^n), right? I think they are all correct. You can apply any legitimate test to any series, whether it is a power function or not.

3. Feb 13, 2008

### HallsofIvy

Staff Emeritus
The ratio and root test work for any series, as Dick said, but it may be difficult to calculate the ratios or roots. It is when you have products or powers, as in power series, that they are easy to apply.

4. Feb 13, 2008

### end3r7

You are correct Dick.

And thanks for the advice guys. =D