“…In [1,2], Arnold, Brezzi, Cockburn, and Marini provided a unified error analysis of DG methods for linear elliptic boundary value problems of second-order and succeeded in building a bridge between these two families of DG methods, establishing a framework to understand their properties, differences and the connections between them. In [21], numerous DG methods were extended for solving elliptic variational inequalities of second-order, and a priori error estimates were established, which are of optimal order for linear elements. DG methods for the Signorini problem and a quasistatic contact problem were also studied in [22,23], respectively.…”