I've been thinking about the calculation of inverse functions through Taylor series expansions.
My hypothesis was that if we had: $$\ f(x) =\sum_{n=0}^\infty \frac{(x-x_0)}{n!}f^{n}(x_0),$$ then $$\ f^{-1}(x) =\sum_{n=0}^\infty \frac{(x-x_0)}{n!}({f^{-1}})^{(n)}(x_0).$$
Here it'd be possible to calculate the derivatives $\ ({f^{-1}})^{(n)}(x_0)$ through the inverse function derivation theorem but for this to work we should have the derivative of the inverse of$\ f$ equal the inverse of the derivative of $\ f$, i.e. $$\ (f^{-1})' = (f')^{-1} \quad (1).$$ In general this does not hold; take for example $\ f(x) = x^2 $.
To formulate my question, assume we have a differentiable bijective map $\ f:A\rightarrow B$ with bijective derivative. Its inverse $\ f^{-1}$ is also a differentiable bijection. Assume the derivative of the inverse is also bijective.
To try and find a function for which$\ (1) $ holds, I was able to deduce (from$\ (1) $) that if such a function exists, we must have $$\ f(x) = (f' \circ f')(x) \quad (2).$$
Clearly this does not hold for any polynomial nor (I'm pretty sure) for any other elementary functions. So, does there exist a function for which this condition holds?
Deriving (2):
$\ (f')^{-1}(x) = (f^{-1})'(x) = \frac{1}{(f' \circ f^{-1})(x)} \iff (f' \circ f^{-1})(x) = \frac{1}{(f')^{-1}(x)}$. Then mapping by $\ f$ from the right gives $\ f'(x)=\frac{1}{(f')^{-1}(f(x))}$ and mapping by $\ \frac{1}{f'}$ from the left $\ \frac{1}{f(x)} = (\frac{1}{f'} \circ f')(x) \iff f(x)= \frac{1}{(\frac{1}{f'} \circ f')(x)}=\frac{1}{\frac{1}{(f' \circ f')(x)}}=(f' \circ f')(x)$
No comments:
Post a Comment