The package uses the preconditioned conjugate gradient method to solve the symmetric positive-deﬁnite linear system
and implements several stopping criteria based on lower and upper bounds of the A-norm of the error. If is the preconditioning matrix, the routine actually solves the preconditioned system
with and and recovers the solution .
Reverse communication is used. Control is returned to the user for preconditioning operations and the products of with a vector .