Space-Time Petrov-Galerkin FEM for Fractional Diffusion Problems

We present and analyze a space-time Petrov-Galerkin finite element method for a time-fractional diffusion equation involving a Riemann-Liouville fractional derivative of order alpha is an element of(0, 1) in time and zero initial data. We derive a proper weak formulation involving different solution and test spaces and show the inf-sup condition for the bilinear form and thus itswell-posedness. Further, we develop a novel finite element formulation, show the well-posedness of the discrete problem, and derive error bounds in both energy and L-2 norms for the finite element solution. In the proof of the discrete inf-sup condition, a certain nonstandard L-2 stability property of the L-2 projection operator plays a key role. We provide extensive numerical examples to verify the convergence analysis.