The $n^\text{th}$ derivative of $f(x)=\frac{1}{1+x^2}$

calculusderivativestaylor expansion

I want the taylor series expansion around some value $a$ of the function $f(x)=\frac{1}{1+x^2}$. I used the general formula
\begin{eqnarray}
f(x) = f(a) + \sum_{n=1}^\infty \frac{f^{(n)}(a)}{n!}(x-a)^n
\end{eqnarray}
But unfortunately, I cannot compute any general formula for $f^{(n)}(a)$. The first derivative is
$$ f^{(1)}(x)= -\frac{2x}{(1+x^2)^2}.$$The second derivative is
$$ f^{(2)}(x)= \frac{6x^2-2}{(1+x^2)^3}.$$The third derivative is
$$ f^{(3)}(x)= \frac{24x(x^2-1)}{(1+x^2)^4}.$$The fourth derivative is
$$ f^{(4)}(x)= -\frac{24(5x^4-10x^2+12)}{(1+x^2)^5}$$. The fifth derivative is
$$ f^{(5)}(x)= \frac{240x(3x^4-10x^2+3)}{(1+x^2)^5}$$.

What is the $n$-th derivative of the function for working with the above taylor series which I want to use to prove something?

Best Answer

$$2f(x)=\frac1{1+ix}+\frac1{1-ix}.$$ Therefore $$2f^{(n)}(x)=\frac{(-i)^nn!}{(1+ix)^{n+1}}+\frac{i^nn!}{(1-ix)^{n+1}}.$$

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