Correction to "What is a fractional derivative?" by Ortigueira and Machado [Journal of Computational Physics, Volume 293, 15 July 2015, Pages 4-13. Special issue on Fractional PDEs].

There is a debate among contemporary mathematicians about what it really means by a fractional derivative. The question arose as a consequence of introducing a 'new' definition of a fractional derivative in 1. In a reply, Ortigueira and Machado 2 came up with several very important criteria to determine whether a given derivative is a fractional derivative. According to their criterion, the new fractional derivative, called conformable fractional derivative, introduced by Khalil et al. 1 turns out not to be a fractional derivative, but rather a controlled or conformable derivative. In proving the claim the authors in 2 use an example 2, p. 6. It turns out that the explanation given there needs some corrections and it is the sole purpose of this note.