APRILE 16 Introduction to mixed problems. The mixed formulation of Laplace problem 16 The Stokes problem and its mixed formulation. Abstract framework for mixed formulations 17 Standard and mixed formulations (symmetric case): associated energy and minimization; saddle point 17 Abstract setting: towards the Banach closed range theorem 20 Banach closed range theorem. Existence and uniqueness of the solution of mixed formulations (simplified case) 20 Existence and uniqueness of the solution of mixed formulations (general case: necessary and sufficient conditions) 23 Verification of the sufficient conditions for Laplace and Stokes problems 23 Inf-sup conditions. A priori estimates for the continous solution. The case of B not surjective 24 Finite element approximation of mixed problems. Existence, uniqueness, and stability of the discrete solution (discrete inf-sup and conditions on the discrete matrix) 24 Consistency and stability implies convergence. Convergence and error estimate for the discrete solution. Commuting diagrams 27 Example: discrete divergence and continuous divergence. Error estimates: discrete kernel included in the continuous kernel 27 Mixed finite elements for the Laplace problem: P1-P0 does not work MAGGIO 4 Properties of functional spaces defined on the union of two subdomains (continuity of traces for H1 and Hdiv) 4 Definition of Raviart-Thomas spaces on simplices and first properties 7 Raviart-Thomas spaces: interpolation, count of degrees of freedom, unisolvence 7 Introduction to the Piola transform 11 Piola transform and its properties 11 Approximation properties of Raviart-Thomas spaces 14 Global interpoolation for Raviart-Thomas spaces and approximation properties. Fortin operator 14 Inf-sup condition for Raviart-Thomas spaces. Stability and error estimates for the mixed Laplacian approximated via Raviart-Thomas scheme. Introduction to the approximation of the Stokes problem 15 Remarks on P1-P0 and P1-P1 elements. Spurious pressure modes. The P2-P0 element 15 Fortin operator for the P2-P0 element. The SMALL element (2D and 3D). The MINI element 18 Crouzeix-Raviart element. Q2-P1 element and higher order families 18 Taylor-Hood element (sketch on macroelement condition). Overview on the approximation of eigenvalue problems