# Inverse and derivative

• Aug 10th 2009, 04:43 PM
NonCommAlg
Inverse and derivative
Give an example of a differentiable function $\displaystyle f: (0, \infty) \longrightarrow \mathbb{R}$ such that $\displaystyle f '(x)=f^{-1}(x), \ \forall x > 0.$ Can you prove that there are no other examples?
• Aug 10th 2009, 07:22 PM
luobo
Quote:

Originally Posted by NonCommAlg
Give an example of a differentiable function $\displaystyle f: (0, \infty) \longrightarrow \mathbb{R}$ such that $\displaystyle f '(x)=f^{-1}(x), \ \forall x > 0.$ Can you prove that there are no other examples?

Here is one example:

Let $\displaystyle f(x)=ax^n$, then
$\displaystyle f'(x)=anx^{n-1}$
$\displaystyle f(f'(x))=f(anx^{n-1})=a(anx^{n-1})^n=a^{n+1}n^nx^{n(n-1)}=x$

Therefore
$\displaystyle n(n-1)=1, a^{n+1}n^n=1$
$\displaystyle n_1=\frac{1+\sqrt{5}}{2}$, $\displaystyle a_1=\left(\frac{1+\sqrt{5}}{2}\right)^{\frac{1-\sqrt{5}}{2}}$
$\displaystyle n_2=\frac{1-\sqrt{5}}{2}$, $\displaystyle a_2=\left(\frac{1-\sqrt{5}}{2}\right)^{\frac{1+\sqrt{5}}{2}}$ (This one is not real).

So,
$\displaystyle f(x)=\left(\frac{1+\sqrt{5}}{2}\right)^{\frac{1-\sqrt{5}}{2}}x^{\frac{1+\sqrt{5}}{2}}$
• Aug 10th 2009, 07:44 PM
simplependulum
Quote:

Originally Posted by luobo
Here is one example:

Let $\displaystyle f(x)=ax^n$, then
$\displaystyle f'(x)=anx^{n-1}$
$\displaystyle f(f'(x))=f(anx^{n-1})=a(anx^{n-1})^n=a^{n+1}n^nx^{n(n-1)}=x$

Therefore
$\displaystyle n(n-1)=1, a^{n+1}n^n=1$
$\displaystyle n_1=\frac{1+\sqrt{5}}{2}$, $\displaystyle a_1=\left(\frac{1+\sqrt{5}}{2}\right)^{\frac{1-\sqrt{5}}{2}}$
$\displaystyle n_2=\frac{1-\sqrt{5}}{2}$, $\displaystyle a_2=\left(\frac{1-\sqrt{5}}{2}\right)^{\frac{1+\sqrt{5}}{2}}$ (This one is not real).

So,
$\displaystyle f(x)=\left(\frac{1+\sqrt{5}}{2}\right)^{\frac{1-\sqrt{5}}{2}}x^{\frac{1+\sqrt{5}}{2}}$

Is it the unique function for this ?
• Aug 10th 2009, 07:58 PM
NonCommAlg
Quote:

Originally Posted by simplependulum
Is it the unique function for this ?

yes! luobo's example, i.e. $\displaystyle f(x)=\varphi^{1 - \varphi}x^{\varphi},$ where $\displaystyle \varphi=\frac{1+\sqrt{5}}{2},$ is the only function that satisfies the condition. (Cool) now the question is: why?