What is really a fractional derivative? When can we affirm it is? Shouldn't we require that they recover the classic when the order becomes integer?
The word fractional appears in a lot of contexts. It became like a fashion. It is clear that Grunwald-Letnikov, Riemann-Louville, Caputo, or Riesz are fractional derivatives, but it is clear for me that the so called fractional Fourier transform is not really fractional. And like this one there are others. Personally, I believe that the most correct way of going into the fractional derivative is the Grunwald-Letnikov, because all the others can be deduced from it. Besides it is the most suitable for numerical computations.