Abstract
This paper investigates a unique and stable numerical approximation of the Riemann–
Liouville Fractional Derivative. We utilize diagonal norm finite difference-based time
integration methods within the summation-by-parts framework. The second-order accurate
discretizations developed in this study are proven to possess eigenvalues with strictly
positive real parts for non-integer orders of the fractional derivative. These results lead to
provably invertible, fully discrete approximations of Riemann–Liouville derivatives.