We present a new stability and convergence analysis for the spatial discretisation of a time-fractional Fokker-Planck equation in a polyhedral domain, using continuous, piecewise-linear, finite elements. The forcing may depend on time as well as on the spatial variables, and the initial data may have low regularity. Our analysis uses a novel sequence of energy arguments in combination with a generalised Gronwall inequality. Although this theory covers only the spatial discretisation, we present numerical experiments with a fully-discrete scheme employing a very small time step, and observe results consistent with the predicted convergence behaviour.Keywords Time-dependent forcing · stability · non-smooth solutions, optimal convergence analysis Mathematics Subject Classification (2010) 65M12 · 65M15 · 65M60 · 65Z05 · 35Q84 · 45K05
IntroductionWe consider the spatial discretisation via Galerkin finite elements of a time-fractional Fokker-Planck equation [1,13], ∂ t u − ∇ · ∂ 1−α t κ α ∇u − F∂ 1−α t u = 0 for x ∈ Ω and 0 < t < T ,