Reliable and efficient a posteriori error estimates of DG methods for a   frictional contact problem
  A posteriori error estimators are studied for discontinuous Galerkin methods for solving a frictional contact problem, which is a representative elliptic variational inequality of the second kind. The estimators are derived by relating the error of the variational inequality to that of a linear problem. Reliability and efficiency of the estimators are shown. 
Fei Wang and Weimin Han
math.NA