Sunday, 6 August 2017

power series - Existence of a function whose derivative of inverse equals the inverse of the derivative

I've been thinking about the calculation of inverse functions through Taylor series expansions.



My hypothesis was that if we had:  f(x)=n=0(xx0)n!fn(x0),

then  f1(x)=n=0(xx0)n!(f1)(n)(x0).

Here it'd be possible to calculate the derivatives  (f1)(n)(x0) 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.  (f1)=(f)1(1).
In general this does not hold; take for example  f(x)=x2.



To formulate my question, assume we have a differentiable bijective map  f:AB with bijective derivative. Its inverse  f1 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)=(ff)(x)(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)=(f1)(x)=1(ff1)(x)(ff1)(x)=1(f)1(x). Then mapping by  f from the right gives  f(x)=1(f)1(f(x)) and mapping by  1f from the left  1f(x)=(1ff)(x)f(x)=1(1ff)(x)=11(ff)(x)=(ff)(x)

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