# Math Help - A Power Series

1. ## A Power Series

Hi everyone ! I need a help with this stuff

Show that a power series c(sub n)x^n has the same radius of convergence as c(sub n+m)x^n, for any positive integer m.

Thanks.

2. Originally Posted by drpawel
Hi everyone ! I need a help with this stuff

Show that a power series c(sub n)x^n has the same radius of convergence as c(sub n+m)x^n, for any positive integer m.

Thanks.
Note that making this change will only make a constant change in the series. See if you can extrapolate a proof from that. If not check back and I will present my solution.

3. Actually,I figured out that it is constant change,but still I can't find the way to show it.
Can you help me?

4. Originally Posted by drpawel
Hi everyone ! I need a help with this stuff

Show that a power series c(sub n)x^n has the same radius of convergence as c(sub n+m)x^n, for any positive integer m.

Thanks.
Suppose that sum series $\sum{c_nx^n}$ converges $\forall{x}\in(-\delta,\delta)$. This implies that $\lim_{n\to\infty}\sqrt[n]{|c_nx^n|}<1~\forall{x}\in(-\delta,\delta)$

Now we are looking at $\sum{c_{n+m\in\mathbb{N}}x^n}$. So using the ratio test we need to find the value of x such that $\lim_{n\to\infty}\sqrt[n]{|c_{n+m}x^n|}<1$. Now letting $n+m=z$ we have $\lim_{z\to\infty}\sqrt[z-m]{c_zx^{z-m}}=|x|\lim_{z\to\infty}\sqrt[z-m]{c_z}\sim|x|\lim_{x\to\infty}\sqrt[z]{c_z}<1~\forall{x}\in(-\delta,\delta)\quad\blacksquare$

5. Originally Posted by drpawel
Hi everyone ! I need a help with this stuff

Show that a power series c(sub n)x^n has the same radius of convergence as c(sub n+m)x^n, for any positive integer m.

Thanks.
both series are convergent at x = 0. for $x \neq 0$ let $p(x)=c_0 + c_1x + \cdots + c_{m-1}x^{m-1}.$ now the claim trvially follows from this identity: $\sum_{n=0}^{\infty}c_nx^n = x^m\sum_{n=0}^{\infty}c_{n+m}x^n +p(x). \ \ \Box$