Description:

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 but it may be definite or indefinite or singular. 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.
MINRES is really solving the least-squares problem minimize ||(A - sI)x
- b||.