A simple finite element method for boundary value problems with a Riemann–Liouville derivative

We consider a boundary value problem involving a Riemann–Liouville fractional derivative of order α∈(3/2,2)α∈(3/2,2) on the unit interval (0,1)(0,1). The standard Galerkin finite element approximation converges slowly due to the presence of singularity term xα−1xα−1 in the solution representation. In this work, we develop a simple technique, by transforming it into a second-order two-point boundary value problem with nonlocal low order terms, whose solution can reconstruct directly the solution to the original problem. The stability of the variational formulation, and the optimal regularity pickup of the solution are analyzed. A novel Galerkin finite element method with piecewise linear or quadratic finite elements is developed, and L2(D)L2(D) error estimates are provided. The approach is then applied to the corresponding fractional Sturm–Liouville problem, and error estimates of the eigenvalue approximations are given. Extensive numerical results fully confirm our theoretical study.