Lowest-order Nonstandard Finite Element Methods for Time-Fractional Biharmonic Problem

In this work, we consider an initial-boundary value problem for a time-fractional biharmonic equation in a bounded convex polygonal domain in R² with clamped boundary conditions. After establishing the well-posedness, we focus on some regularity results of the solution with respect to the regularity of the problem data. The spatially semidiscrete scheme covers several popular lowest-order piecewise-quadratic finite element schemes, namely, Morley, discontinuous Galerkin, and C⁰ interior penalty methods, and includes both smooth and nonsmooth initial data. Optimal order error bounds with respect to the regularity assumptions on the data are proved for both homogeneous and nonhomogeneous problems. The numerical experiments validate the theoretical convergence rate results.