$\mathit{\text{A}}\mathit{\text{priori}}$ and $\mathit{\text{a}}\mathit{\text{posteriori}}$ error identities for the scalar Signorini problem

In this paper, on the basis of a (Fenchel) duality theory on the continuous level, we derive an $\mathit{\text{a}}\mathit{\text{posteriori}}$ error identity for arbitrary conforming approximations of the primal formulation and the dual formulation of the scalar Signorini problem. In addition, on the basis of a (Fenchel) duality theory on the discrete level, we derive an $\mathit{\text{a}}\mathit{\text{priori}}$ error identity that applies to the approximation of the primal formulation using the Crouzeix-Raviart element and to the approximation of the dual formulation using the Raviart-Thomas element, and leads to quasi-optimal error decay rates without imposing additional assumptions on the contact set and in arbitrary space dimensions.