Guaranteed, locally space-time efficient, and polynomial-degree robust a posteriori error estimates for high-order discretizations of parabolic problems

We consider the a posteriori error analysis of approximations of parabolic problems based on arbitrarily high-order conforming Galerkin spatial discretizations and arbitrarily high-order discontinuous Galerkin temporal discretizations. Using equilibrated ux reconstructions, we present a posteriori error estimates for a norm composed of the L²(H¹)"H¹(H^-1)-norm of the error and the temporal jumps of the numerical solution. The estimators provide guaranteed upper bounds for this norm without unknown constants. Furthermore, the efficiency of the estimators with respect to this norm is local in both space and time, with constants that are robust with respect to the mesh-size, time-step size, and the spatial and temporal polynomial degrees. We further show that this norm, which is key for local space-time effciency, is globally equivalent to the L²(H¹) "H¹(H^-1)-norm of the error, with polynomial-degree robust constants. The proposed estimators also have the practical advantage of being robust with respect to refinement and coarsening between the time steps.