1. Babuska, I., Aziz, A.K.: Mathematical foundations of the finite element method. New York: Academic Press 1972
2. Babuska, I., Oden, J.T., Lee, J.K.: Mixed hybrid finite element approximation of second order elliptic boundary value problems. Comput. Methods Appl. Mech. Engrg.14, 1?23 (1978)
3. Brezzi, F.: On the existence, uniqueness and application of saddle point problems arising from lagrange multipliers. R.A.I.R.O.8, 129?150 (1975)
4. Ciarlet, P.G.: The finite element method for elliptic problems. Amsterdam: North Holland Publishing 1977
5. Fix, G.J., Gunzburger, M.D., Nicolaides R.A.: Theory and applications of mixed finite element methods. Constructive approaches to mathematical models. New York: Academic Press, pp. 375?393, 1979