would be possible to define the fractional derivative (and integral) as
$$ D^{a}f(x)=F.P\frac{1}{\Gamma(-a)}\int_{c}^{x}dt \frac{f(x)-f(t)}{(x-t)^{1+a}}$$
here c ,a are real constant (a can be negative or positive) is this a formal good definition for the fractional derivative ?? :D , here F.P means the finite part of the integral