โ{128}{156}Well posedโ{128}{157} boundary value problems -- Existence principle -- The function spaces and Hm -- The trace operator. Sobolev and Ehrling lemmas -- Elliptic linear systems. Interior regularity -- Existence of local solutions for elliptic systems -- Semiweak solutions of BVP for elliptic systems -- Regularity at the boundary: preliminary lemmas -- Regularity at the boundary: tangential derivatives -- Regularity at the boundary: final results -- The classical elliptic BVP of Mathematical physics: 2nd order linear PDE. -- The classical elliptic BVP of Mathematical Physics: Linear Elastostatics -- The classical elliptic BVP of Mathematical Physics: Equilibrium of thin plates -- Strongly elliptic operators. Gยปrding inequality. Eigenvalue problems -- Eigenvalue problems. The Rayleigh-Ritz method -- The Weinsteinโ{128}{148}Aronszajn method -- Construction of the intermediate operators -- Orthogonal invariants of positive compact operators -- Upper approximation of the eigenvalues of a PCO. Representation of orthogonal invariants -- Explicit construction of the Green's matrix for an elliptic system -- Erratum