“…Finally, by combing the results above, we can prove the optimal approximation capabilities for the proposed IFE space through the following error estimation for the global interpolation operator.Theorem There exists a constant C independent of the interface location such that the following estimate holds for every : where | · | 1, h , Ω and | · | 2, h , Ω are the usual discrete semi‐norms defined according to the mesh . Proof By putting and together over all the elements T , we have Then using the inequality for any w ∈ W 1, p (Ω) from , we have .…”