“…A posteriori error estimation for nonconforming finite element methods with application to second-order elliptic problems has recently seen significant progress and is still the subject of sustained research efforts, see e.g. [9,34,51,55,33,1,18,42,41,27,24] for dG methods and [16,35,2,17,37] for mixed finite element methods. We also refer the reader to [36,18,3,28,22] and references therein for the presentation of unifying frameworks on the topic.…”