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
Open question
2022-08-19
Derivative of at points when I am interested in understanding the general behavior of the derivative for
at points where For example, if we have
The general formula
breaks down when , though as the example above shows the derivative may still exist there. I am not sure what the proper assumptions should be. Tentatively, take for all in the relevant domain, so that (I believe) we have
when 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) where takes on negative values and where is not constant? In other words, for cases of are the only functions worth considering of the form for constants ? EDIT: I want the function(s) to be real, although arguments using complex numbers are of course permissible.
Answer & Explanation
Madilyn Dunn
Beginner2022-08-20Added 16 answers
I would imagine that for negative f<0 you could write:
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.