“…where F K,e is the approximated flux trough the edge e of the element K, and E K is the set of edges of the element K. Note that F K,e represents both standard fluxes and, for faces on the boundary to lower-dimensional objects, projections of mortar fluxes. Now the fluxes F K,e can be extended using RTN 0 on each element for each subdomain [18,50]. Finally, we can construct u h by collecting all the tangential and normal fluxes, i.e., u h = [u i,h , λ ij,h ].…”