I read from Leonard Mlodinow's Feynman's Rainbow that Feynman has defined the concept of a half-derivative, an operation $g$ on a function $f$ such that $$g(g(f))=f'.$$ I wonder if it is possible to develop a general rule of finding the half-derivative of any "half-differentiable" function just as finding the derivative of any differentiable function? Furthermore, can we generalize our answers to the previous questions to $\frac{1}{n}$-derivatives? Can we even defind irrational-derivatives?
Thanks in advance for any help.