Monotonicity of functions and sign changes of their Caputo derivatives

Original · EN

It is well known that a continuously differentiable function is monotone in an interval [a,b] if and only if its first derivative does not change its sign there. We prove that this is equivalent to requiring that the Caputo derivatives of all orders α∈ (0,1) with starting point a of this function do not have a change of sign there. In contrast to what is occasionally conjectured, it not sufficient if the Caputo derivatives have a constant sign for a few values of α∈ (0,1) only.