Another view for a posteriori error estimates for variational inequalities of the second kind

In this paper, we give another view to understand a posteriori error analysis for finite element solutions of elliptic variational inequalities of the second kind. This point of view makes it simpler to derive reliable error estimators in solving variational inequalities of the second kind from the theory for related linear variational equations. Reliable residual-based and gradient recovery-based estimators are deduced. Efficiency of the estimators is also proved.