“…Despite the aforementioned progress, the corresponding results for the variable coefficient fractional diffusion, advection, reaction equations are largely missing. There exists some recent work on Petrov-Galerkin approximations to two-sided fractional diffusion, reaction equations [15,17], onesided fractional diffusion, advection, reaction equations [8,12,20], and two-sided fractional diffusion, advection, reaction equations [36], all with constant diffusivity coefficients. To the best of our knowledge, the only available result for the Petrov-Galerkin method applied to variable coefficient fractional diffusion problems is [29], in which the weak coercivity in the sense of inf-sup condition was proved for the one-sided variable coefficient fractional diffusion operator, i.e., Lα r with r = 1.…”