# Homework Help: Radius of convergence and 2^1/2

1. Jan 16, 2013

### bedi

1. The problem statement, all variables and given/known data

Suppose c_n is the digit in the nth place of the decimal expansion of 2^1/2. Prove that the radius of convergence of $\sum{c_n x^n}$ is equal to 1.

2. Relevant equations

3. The attempt at a solution

What I want to show is that limsup |c_n|^1/n = 1. Clearly for any c_n, (c_n)^1/n ≥ 1. So we have sup (c_n)^1/n ≥ 1 for any n. It is also easy to see that sup (c_n)^1/n ≤ n^1/n for n large enough, as c_n is at most 9. So by the Sandwich lemma limsup |c_n|^1/n = 1.

Correct?

2. Jan 16, 2013

### jbunniii

Why is this clearly true? Are you saying that the decimal expansion of $\sqrt{2}$ doesn't contain any zero digits?
Yes, this part is true, because there are infinitely many nonzero $c_n$'s. (Proof?)

: Be careful about the notation. I think you meant
$$\sup \{(c_m)^{1/m} : m \geq n\} \geq 1 \textrm{ for any } n$$
and therefore
$$\limsup (c_n)^{1/n} \geq 1$$
Yes, that looks OK, assuming you have already proved that $\lim_{n\rightarrow \infty} n^{1/n} = 1$.

Last edited: Jan 16, 2013