  We introduced and analyzed robust recovery-based a posteriori error
estimators for various lower order finite element approximations to interface
problems in [9, 10], where the recoveries of the flux and/or gradient are
implicit (i.e., requiring solutions of global problems with mass matrices). In
this paper, we develop fully explicit recovery-based error estimators for lower
order conforming, mixed, and non- conforming finite element approximations to
diffusion problems with full coefficient tensor. When the diffusion coefficient
is piecewise constant scalar and its distribution is local quasi-monotone, it
is shown theoretically that the estimators developed in this paper are robust
with respect to the size of jumps. Numerical experiments are also performed to
support the theoretical results.
