CONTENTS: Implementation of a conjugate-gradient type method
for solving sparse linear equations:
Solve Ax = b or (A - sI)x = b.
The matrix A - sI must be symmetric and nonsingular,
but it may be definite or indefinite.
The scalar s is a shifting parameter -- it may be any number.
The method is based on Lanczos tridiagonalization.
You may provide a preconditioner, but it must be positive definite.
If A - sI is symmetric but singular,
use MINRES.
If A is unsymmetric,
use LSQR.