“…This operator is also known as the generator form of fractional derivatives [30,38], or a Lévy-type generator [10]. (iii) Other particular cases include the fractional derivatives of variable order, which are obtained by taking ρ as the function ρ(t, r) = −r −1−α(t) /Γ(−α(t)) with a suitable function α(t) : R → (0, 1) [27], and tempered Lévy kernels ρ(t, r) = −e −λr r −1−α /Γ(−α), α ∈ (0, 1), λ > 0, [11,51].…”