The operator and the preconditioner must be symmetric for this method. The
preconditioner must be POSITIVE-DEFINITE and the operator POSITIVE-SEMIDEFINITE.
Support only for left preconditioning.
- Magnus R. Hestenes and Eduard Stiefel, Methods of Conjugate Gradients for Solving Linear Systems,
Journal of Research of the National Bureau of Standards Vol. 49, No. 6, December 1952 Research Paper 2379