# Radii of convergence of a power series

• Jan 8th 2006, 10:54 AM
TexasGirl
Radii of convergence of a power series
How do I compute the radii of convergence of the power series Σanx^n

(from n=0 to infinity)

with coefficients an=n and an=1/n
• Jan 8th 2006, 11:27 AM
ThePerfectHacker
Quote:

Originally Posted by TexasGirl
How do I compute the radii of convergence of the power series Σanx^n

(from n=0 to infinity)

with coefficients an=n and an=1/n

Use the ratio test for the power series. With $a_k=k$ thus, $\lim_{k\rightarrow \infty}|\frac{k+1}{k}|=1$ Thus, the radius of convergence is the the reciprocal of that thus $1/1=1$. However, the interval of convergence is $-1. For $x=-1,1$ this power series diverges.

Now for the second problem, $a_k=1/k, k>1$ use the ratio test for power series again to get $\lim_{k\rightarrow \infty}|\frac{k}{k+1}|=1$ Thus, the radius of converges is 1, thus for $-1 converges absolutely. Checking the endpoint (because the ratio test is inconclusive for when its limit is one), we have
$\sum^\infty_{k=1} (-1)^k 1/k$ but this is the alternating-harmonic series thus it converges. For the second possibility we have that $\sum^\infty_{k=1} 1/k$ but this is the harmonic series thus it diverges. Thus, the interval of convergence for the second power series is $-1\leq x<1$
• Jan 9th 2006, 01:35 PM
TexasGirl
One More Radius of Convergence Question...
Building from the same power series, if I take q, a nonzero element of C, and put it into the series so that I have the sum of an(q^n)(x^n), can I still use the ratio test in order to find the radius of convergence? Is an the coefficient?
• Jan 9th 2006, 02:54 PM
ThePerfectHacker
Quote:

Originally Posted by TexasGirl
Building from the same power series, if I take q, a nonzero element of C, and put it into the series so that I have the sum of an(q^n)(x^n), can I still use the ratio test in order to find the radius of convergence? Is an the coefficient?

I do not understand what you are asking?
• Jan 9th 2006, 03:14 PM
TexasGirl
Here is the question I have to answer:

Assume that the power series ∑an(x^n), where n=0 to infinity, has radius of convergence p, where p is a nonnegative real number or stands for the symbol ∞. Let q be an element of C, q≠0. Compute the radius of convergence of ∑an(q^n)(x^n) (n=0 to infinity).
• Jan 10th 2006, 01:56 PM
ThePerfectHacker
As I understand it, its radius of convergence would be $p/q$ Explanation:
If $\sum^{\infty}_{k=1}a_kx^k$ has radius of convergence of $p$ then by the ratio test for power series $\lim_{k\rightarrow \infty}|\frac{a_{k+1}}{a_k}|=1/p$ because as I said it is the reciprocal of the limit. Thus, the new infinite series given by $\sum^{\infty}_{k=1}a_kq^kx^k$ then by the ratio test $\lim_{k\rightarrow \infty}|\frac{a_{k+1}q^{k+1}}{a_kq^k}|=\frac{q}{p}$ thus, the radius of convergent is the reciprocal thus $\frac{p}{q}$
• Jan 11th 2006, 10:47 AM
TexasGirl
thanks again...
much appreciated... :)
• Jan 11th 2006, 03:21 PM
ThePerfectHacker
You are welcome.