“…For the one-dimensional two-sided FDEs, there are several available numerical methods, for example, the finite difference method [26,37], the finite element method [11,38], and the spectral method [19,43,24,22,10,23] and references therein. In the early works, the emphasis was on obtaining high accuracy by ignoring the issue of low regularity of the solution of FDEs, i.e., assuming that the solution is smooth, for example, [11,19,37,24]. However, solutions of fractional boundary value problems have endpoints singularities that limit the convergence rate of numerical discretizations significantly.…”