1. Axelsson, O.: On Preconditioning and Convergence Acceleration in Sparse Matrix Problems. CERN 74-10 (1974).
2. Blum, E.K.: Numerical Analysis and Computation: Theory and Practice. pp. 280?290. Addison-Wesley Publishing Company 1972
3. Cline, A.K.: Several Observations on the Use of Conjugate Gradient Methods. ICASE, Report No. 76-22 (1976)
4. Concus, P., Golub, G.H., O'Leary, D.P.: A Generalized Conjugate Gradient Method for the Numerical Solution of Elliptic Partial Differential Equations. In: Sparse Matrix Computations (J. R. Bunch and D.J. Rose, eds.). New York, London: Academic Press 1976
5. Dupont, T., Kendall, R.P., Rachford, Jr., H.H.: An Approximate Factorization Procedure for Solving Self-Adjoint Elliptic Difference Equations. SIAM J. Numer. Anal.5, 559?573 (1968)