Derivative of f(x)^(g(x)) at points when f(x)=0 I am interested in understanding the general behavior of the derivative for f(x)^(g(x)) at points where f(x)=0. For example, if f^g=x^n we have (d)/(dx)f^g(0)={(0,n>=1),(+-oo, n<1):} The general formula (f^g)′=f^g(g′ln|f|+g(f′)/(f)) breaks down when f=0, though as the example above shows the derivative may still exist there. I am not sure what the proper assumptions should be. Tentatively, take f(x)^g(x)>=0 for all x in the relevant domain, so that (I believe) we have ln(f(x)^(g(x)))=g(x)ln|f(x)| when f is strictly positive and undefined otherwise.Also, is there any non-trivial example of a well-defined function (meaning "nice", as in differentiable almost everywhere) f^g where f takes on negative values and where g is not constant? In other

Bierlehre59

Bierlehre59

Open question

2022-08-19

Derivative of f ( x ) g ( x ) at points when f ( x ) = 0
I am interested in understanding the general behavior of the derivative for
f ( x ) g ( x )
at points where f ( x ) = 0
For example, if f g = x n we have
d d x f g ( 0 ) = { 0 n 1 ± n < 1
The general formula
( f g ) = f g ( g ln | f | + g f f )
breaks down when f = 0, though as the example above shows the derivative may still exist there.
I am not sure what the proper assumptions should be. Tentatively, take f ( x ) g ( x ) 0 for all x in the relevant domain, so that (I believe) we have
ln ( f ( x ) g ( x ) ) = g ( x ) ln | f ( x ) |
when f is strictly positive and undefined otherwise.
Also, is there any non-trivial example of a well-defined function (meaning "nice", as in differentiable almost everywhere) f g where f takes on negative values and where g is not constant? In other words, for cases of f < 0 are the only functions worth considering of the form f c for constants c?
EDIT: I want the function(s) to be real, although arguments using complex numbers are of course permissible.

Answer & Explanation

Madilyn Dunn

Madilyn Dunn

Beginner2022-08-20Added 16 answers

I would imagine that for negative f<0 you could write:
f g = ( 1 ) g | f | g = e ± i π g | f | g = ( cos ( π g ) ± i sin ( π g ) ) | f | g
So in general it is a complex valued function and the argument (or the phase) of the function f^g is independent on f. There are also two possible choices for the imaginary part of the function. Once you make that choice I would imagine that there are many examples which are differentiable (differentiable f and g should work) provided you don't mind having a complex function of one real variable.

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