First order differential equation involving inverses My question is to find the solutions to the following

$\frac{df(x)}{dx} = f^{-1} (x)$

where $f^{-1} (x)$ refers to the inverse of the function f. The domain really isn't important, though I am interested in either (-inf, inf) or (0, inf), so if any solutions are known for more restricted domains then they are welcome.
I cannot find any material relating to this type of question in any of my calculus and differential equations textbooks and references; it seems quite unorthodox. Any material which covers this type of diff equation would be wlecome
 A: Here is a partial solution using power series composition.

To solve
$$
f(f'(x))=x\tag{1}
$$
take the derivative of $(1)$
$$
f'(f'(x))\,f''(x)=1\tag{2}
$$
If we can find a point $a\gt0$ so that $f'(a)=a$, we can expand $f'(a+x)$ around $x=0$. Let
$$
g(x)=f'(a+x)-a\tag{3}
$$
Applying $f'(a+x)=a+g(x)$ twice gives
$$
\begin{align}
f'(f'(a+x))
&=f'(a+g(x))\\
&=a+g(g(x))\tag{4}
\end{align}
$$
Differentiating $(3)$ yields
$$
g'(x)=f''(a+x)\tag{5}
$$
Plugging $(4)$ and $(5)$ into $(2)$, we get
$$
g'(x)=\frac1{a+g(g(x))}\tag{6}
$$
Iterating $(6)$ gives
$$
\begin{align}
g(x)
&=\frac xa-\frac{x^2}{2a^4}+\frac{(1+3a)x^3}{6a^8}-\frac{(1+3a+10a^2+15a^3)x^4}{24a^{13}}\\
&+\frac{\left(1+3a+10a^2+30a^3+55a^4+105a^5+105a^6\right)x^5}{120a^{19}}+\dots\tag{7}
\end{align}
$$
Therefore,
$$
\begin{align}
f(x)
&=a+a(x-a)+\frac{(x-a)^2}{2a}-\frac{(x-a)^3}{6a^4}+\frac{(1+3a)(x-a)^4}{24a^8}\\[6pt]
&-\frac{(1+3a+10a^2+15a^3)(x-a)^5}{120a^{13}}\\
&+\frac{\left(1+3a+10a^2+30a^3+55a^4+105a^5+105a^6\right)(x-a)^6}{720a^{19}}+\dots\tag{8}
\end{align}
$$

Comparison with Julián Aguirre's Answer
$$
\begin{align}
f(x)
&=\phi^{-1/\phi}x^\phi\\
&=\phi+\phi(x-\phi)+\frac{\phi-1}2(x-\phi)^2-\frac{5-3\phi}6(x-\phi)^3+\frac{18\phi-29}{24}(x-\phi)^4\\
&-\frac{217-134\phi}{120}(x-\phi)^5+\frac{1219\phi-1972}{720}(x-\phi)^6+\dots\tag{9}
\end{align}
$$
which matches $(8)$ with $a=\phi$.
A: Look for solutions of the form $f(x)=A\,x^a$. The equation reads then
$$
A\,a\,x^{a-1}=\Bigl(\frac{x}{A}\Bigr)^{1/a}.
$$
You can find $A$ and $a$ fron here.
A: It's a bit contrived, but you could take a power series approach. First, write the original ODE as
\begin{equation}
 f(f'(x)) = x. \tag{1}
\end{equation}
Then, assume a solution to $(1)$ exists, and has a power series expansion around $x=0$. That is, we write
\begin{equation}
 f(x) = \sum_{n=0}^\infty a_n x^n\quad\text{and}\quad g(x):= f'(x) = \sum_{m=0}^\infty (m+1)\, a_{m} x^{m}.
\end{equation}
Then, using Faà di Bruno's formula, we can write the power series of the composition
\begin{equation}
 f(g(x)) = \sum_{n=0}^\infty c_n x^n,
\end{equation}
where $c_n$ is expressed in terms of $a_n$ and $n$ through combinatorial expressions. Now, using $(1)$, we see that
\begin{equation}
 c_0 = 0,\; c_1 = 1,\;\text{and}\; c_n = 0\;\text{for}\;n\geq 2.
\end{equation}
This yields a system of equations for $a_n$. Again, it's a bit ugly, but might give some insight.
