# What is the radius of convergence of $\frac{1}{1+x^2}$ at $x_0=0$?

Which is the radius of convergence of Taylor series of $f(x)=\frac{1}{1+x^2}$ around $x_0=0$? I am unable to write down the analitycal expression of all its derivatives. Why is it finite even if $f(x)<\infty$ for all $x\in \mathbb{R}$?

• Are you familiar with the geometric series formula involving $\frac{1}{1-x}$?
– anon
Jan 20, 2013 at 22:32
• @anon, yes, $\frac{1}{1-x}=\sum_n x^n$ Jan 21, 2013 at 16:26

One possibility (which does not require complex analysis) is to use the geometric series, as suggested by anon. We know that for $|r|<1$ $$1+r+r^2+\ldots=\sum_{k=0}^\infty r^k=\frac{1}{1-r}.$$

If we use this with $r=-x^2$, we have $$\frac{1}{1+x^2}=\frac{1}{1-(-x^2)}=\sum_{k=0}^\infty (-x^2)^k=\sum_{k=0}^\infty (-1)^kx^{2k},$$ which is the Taylor series around $x_0=0$ (also known as the Maclaurin series). From the properties of the geometric series, we know that this series converges iff $|-x^2|<1$. This is equivalent to $|x|<1$, so the radius of convergence is one.

• @Did: The "only if" part is also true with my wording. The terms of a convergent series always tend to zero, and if the quotient has magnitude $\geq 1$ the terms do not approach zero. Jan 21, 2013 at 21:41
• Thank you. Can you explain me why it must be $|x|<1$ even if $|f(x)|$ is finite for all $x$? Jan 22, 2013 at 10:15
• @user59051: $|f(x)|$ is indeed finite for all real valued $x$, but as I pointed out in my other answer there are singularities at $x=i$ and $x=-i$. For a complete understanding of what is going on, one needs to know a bit of complex analysis. Jan 22, 2013 at 10:30
• What would happens to the function around $x=\pm1$ if we only use real analysis? I guess nothing's special? Sep 13, 2017 at 19:02
• @Ooker: The function itself will work just fine for all $x\in\mathbb{R}$, but the Maclauring series will not converge if $|x|\geq 1$. Sep 13, 2017 at 21:44

The radius of convergence is the distance to the nearest singularity of the function.

Your function has singularities at $x=\pm i$, so the Maclaurin expansion (Taylor series at zero) of $f(x)=\frac{1}{1+x^2}$ has radius of convergence $r=1$ (the distance from zero to $\pm i$).

If the Taylor series expansion is about $x_0=1$, then the radius of convergence would be $$\min_{x=\pm i}|1-x|=\sqrt{2}.$$

• Since my undergraduate days I wish to see a proof of this fact by pure real analysis arguments, that is, one that does not resort to complex analysis. Jan 21, 2013 at 4:45

$f(\sqrt{-1}) = 1/0$

$f(-\sqrt{-1}) = 1/0$

so the radius of convergence is blocked by these explosions.