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3. Studies in Mathematics and its Applications, Vol. 4;Ciarlet, Philippe G.,1978
4. S. C. Eisenstat, On the Rate of Convergence of the Bergman-Vekua Method for the Numerical Solution of Elliptic Boundary Value Problems, Research Report No. 72-2, Department of Computer Science, Yale University, 1972.
5. Hybrid finite element methods;Fix, George M.;SIAM Rev.,1976