# What is the radius of convergence of

## Homework Statement

z ∈ ℂ

What is the radius of convergence of (n=0 to ∞) Σ anzn?

## Homework Equations

I used the Cauchy-Hardamard Theorem and found the lim sup of the convergent subsequences.

$$a_n = \frac{n+(-1)^n}{n^2}$$

limn→∞ |an|1/n

## The Attempt at a Solution

I think that the radius of convergence is one, i.e. |z| < 1. I figured that the numerator would tend to n with the oscillating 1 and so you'd get $$\frac{n^{1/n}}{n^{2/n}} = 1$$

R = 1/limn→∞ |an|1/n

Dick
Homework Helper

## Homework Statement

z ∈ ℂ

What is the radius of convergence of (n=0 to ∞) Σ anzn?

## Homework Equations

I used the Cauchy-Hardamard Theorem and found the lim sup of the convergent subsequences.

$$a_n = \frac{n+(-1)^n}{n^2}$$

limn→∞ |an|1/n

## The Attempt at a Solution

I think that the radius of convergence is one, i.e. |z| < 1. I figured that the numerator would tend to n with the oscillating 1 and so you'd get $$\frac{n^{1/n}}{n^{2/n}} = 1$$

R = 1/limn→∞ |an|1/n
That's a little informal but it looks fine. You might want worry about what happens at z=1 and z=(-1) if you are concerned about the boundary cases.

That's a little informal but it looks fine. You might want worry about what happens at z=1 and z=(-1) if you are concerned about the boundary cases.
I assume a singularity is there and outside of the disk the series is divergent. Well, I'm leaving out a few bits of information in my "proof" here. I'll state a bit more of the background. What would a more formal proof look like?

Dick