Axioms of adaptivity for separate marking
  Mixed finite element methods with flux errors in $H(div)$-norms and div-least-squares finite element methods require a separate marking strategy in obligatory adaptive mesh-refining. The refinement indicator $\sigma^2(\mathcal T,K)=\eta^2(\mathcal T,K)+\mu^2(K)$ of a finite element domain $K$ in an admissible triangulation $\mathcal T$ consists of some residual-based error estimator $\eta(\mathcal T,K)$ with some reduction property under local mesh-refining and some data approximation error $\mu(K)$. Separate marking means either D\"orfler marking if $\mu^2(\mathcal T) \leq \kappa \eta^2(\mathcal T)$ or otherwise an optimal data approximation algorithm runs with controlled accuracy as established in [Carstensen, Rabus, Math.Comp. 2011; Rabus, J.Numer.Math. 2015].   The axioms are abstract and sufficient conditions on the estimators $\eta(\mathcal T,K)$ and data approximation errors $\mu(K)$ for optimal asymptotic convergence rates. The enfolded set of axioms simplifies \cite{CFP14} for collective marking, treats separate marking established for the first time in an abstract framework, generalizes [Carstensen, Park, SIAM J.Numer.Anal. 2015] for least-squares schemes, and extends [Carstensen, Rabus, Math.Comp. 2011] to the mixed FEM with flux error control in $H(div)$. 
Carsten Carstensen and Hella Rabus
math.NA