Paige and Saunders, "Solution of Sparse Indefinite Systems of Linear Equations", SIAM J. Numer. Anal., vol 12, page 617 (1975) https://doi.org/10.1137/0712047
Eisenstat, Elman & Schultz 1983, Thm 3.3. NB all results for GCR also hold for GMRES, cf. Saad & Schultz 1986 Eisenstat, Stanley C.; Elman, Howard C.; Schultz, Martin H. (1983). "Variational Iterative Methods for Nonsymmetric Systems of Linear Equations". SIAM Journal on Numerical Analysis. 20 (2): 345–357. doi:10.1137/0720023. ISSN0036-1429.
Eisenstat, Elman & Schultz 1983, Thm 3.3. NB all results for GCR also hold for GMRES, cf. Saad & Schultz 1986 Eisenstat, Stanley C.; Elman, Howard C.; Schultz, Martin H. (1983). "Variational Iterative Methods for Nonsymmetric Systems of Linear Equations". SIAM Journal on Numerical Analysis. 20 (2): 345–357. doi:10.1137/0720023. ISSN0036-1429.