“…It is well known that the existence and unicity of the solution of a discrete mixed formulation require a continuous definition of the Lagrange multiplier space in accordance with the displacement space, satisfying the LBB inf-sup condition. Comments and details regarding this question within X-FEM models with contact and friction can be found in [31][32][33]21,34,20,19]. Additional remarks will be exposed in the next sections regarding the unilateral or frictional contact problems.…”