# Showing the power series $\sum_{n = 0}^{\infty} \frac{1}{3 - \cos(x)}$ has a radius of convergence $R > 1$

I want to show that the power series

$$\sum_{n = 0}^{\infty} a_{n}x^{n} = \frac{1}{3 - \cos(x)}$$

has a radius $$R > 1$$? I don't think the ratio test will work here. From Googling, I found the Cauchy-Hadamard Theorem, and I was wondering if I could somehow apply it to this. Can someone please help me? I'm not so familiar with $$\limsup$$.

This can be seen as a heuristic way which may not be necessarily correct.

There is a slightly easier way using the expansion for $$\dfrac{1}{1-x}$$.

$$\dfrac{1}{3 - \cos(x)}=\dfrac{1}{3}\left(\dfrac{1}{1-\dfrac{\cos(x)}{3}}\right)=\dfrac{1}{3}\displaystyle\sum_{n=0}^\infty\left(\dfrac{\cos(x)}{3}\right)^n$$ exists if $$\left|\dfrac{\cos(x)}{3}\right|<1 \implies |\cos(x)|<3$$ which is true for all $$x\in\mathbb{R}$$.

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To find the power series we calculate using Taylor series

$$f(0)=\dfrac{1}{2}, \: f'(x)=\dfrac{-\sin x}{(3-\cos x)^2}\implies f'(0)=0, \: \\f''(x)=\dfrac{(3-\cos x)(-\cos x)+2\sin^2 x(3-\cos x)}{(3-\cos x)^4}\implies f''(0)=\dfrac{1}{4}$$.

Continuing this way we get $$\quad$$ $$\dfrac{1}{3 - \cos(x)}=\dfrac{1}{2}-\dfrac{x^2}{8}+\dfrac{x^4}{24}+\cdots$$

$$\cos(z)=\dfrac{e^{iz}+e^{-iz}}{2}=3\implies e^{2iz}-6e^{iz}+1=0 \implies e^{iz}=\dfrac{6\pm\sqrt{36-4}}{2}=3\pm2\sqrt{2}$$ $$\implies z=2\pi n -i\ln(3\pm2\sqrt{2}),\: n\in\mathbb{Z}$$.

Now since radius of curvature is defined such that the function is analytic in $$\{z: |z - z_0| < r\}$$, so the radius of convergence is at least $$r$$. We infer that if $$f$$ has a pole at $$c$$ then it is analytic for $$|z-z_o|, so radius of curvature is exactly the distance to the closest pole.

Thus $$|z|=|\ln(3+2\sqrt{2})|$$. Thus we see that $$R=|\ln(3+2\sqrt{2})|>1$$.

• that is very clever. – joseph Nov 16 '18 at 13:21
• I don't think this works. The power series of $1/(3- \cos x)$ is not $\frac{1}{3} \sum_n (\frac{\cos x}{3})^n$, because it is not a power series. If you expand it, it does not converge uniformly on the whole real line. Actually the power series has a finite radius of convergence. That's because otherwise it would converge uniformly on the whole $\Bbb C$: but there is the problem that on complex numbers there is a solution for $\cos z=3$. – Crostul Nov 16 '18 at 13:26
• @Crostul: Isnt this true for $x\in\mathbb{R}$ ? How does the power series look like in the case $x\in\mathbb{C}$? – Yadati Kiran Nov 16 '18 at 13:31
• The radius of convergence of that power series is bounded by $\ln (3+2\sqrt 2) = 1.76...$. I think that is exactly the radius of convergence, though I'm not sure. You can see it graphically at wolframalpha.com/input/?i=power+series+of+(3-cos+z)%5E(-1) . – Crostul Nov 16 '18 at 13:33
• @Crostul: I shall check in the case for $x\in\mathbb{C}$. But what about when $x\in\mathbb{R}$ – Yadati Kiran Nov 16 '18 at 13:38

The poles of the function occur at

$$\cos z=3$$ or $$z=\pm i\text{ arcosh}(3)+2k\pi=\pm i\log(3+\sqrt8)+2k\pi.$$

As the radius of convergence is the distance to the closest pole,

$$R=\log(3+\sqrt8)\approx1.76274717.$$

You may also invoke Cauchy's integral theorem. $$f(z)=\frac{1}{3-\cos(z)}$$ is holomorphic over $$|z|\leq\frac{3}{2}$$, since

$$\left|3-\cos(z)\right|\geq 2-\sum_{n\geq 1}\frac{|z|^{2n}}{(2n)!}=3-\cosh(|z|)$$ and $$K=3-\cosh\left(\tfrac{3}{2}\right)>0$$. In particular

$$|a_n|=\left|\frac{1}{2\pi i}\oint_{|z|=\frac{3}{2}}\frac{dz}{z^{n+1}(3-\cos z)}\right|\leq \frac{1}{K\left(\frac{3}{2}\right)^n}$$ by the triangle inequality, and the radius of convergence of $$\sum a_n z^n$$ is at least $$\frac{3}{2}$$.
The same argument works also by replacing $$\frac{3}{2}$$ with $$\frac{7}{4}$$, since $$\cosh\frac{7}{4}$$ still is less than three.