Discrete Extension Operators for Mixed finite element spaces on locally   refined meshes
  The existence of uniformly bounded discrete extension operators is established for conforming Raviart-Thomas and N\'ed\'elec discretisations of $H(div)$ and $H(curl)$ on locally refined partitions of a polyhedral domain into tetrahedra. 
Mark Ainsworth, Johnny Guzm\'an, Francisco-Javier Sayas
math.NA