“…To our knowledge, this is the best result by far for the preasymptotic error estimate of higher-order FEM. Numerous approaches have emerged over the past two decades to reduce the pollution errors, including hp-FEM [9,40,50,51], CIP-FEM [22,46,47,55,59], discontinuous Galerkin method (DG) [23,24,32,49,58], Trefftz methods [30,31,[34][35][36][37][38]42], and multiscale methods [8,29,52]. In this paper, we would like to introduce the higher-order CIP-FEM which offers significant advantages in reducing pollution errors.…”