Introduction - If you have any usage issues, please Google them yourself
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.