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HSL_MI20 Unsymmetric system: algebraic multigrid preconditioner

Given an $n\times n$ sparse matrix $A$ HSL_MI20 has two functions.

1. Given an $n-$vector $z$, HSL_MI20 computes the vector $x=Mz$, where $M$ is an algebraic multigrid (AMG) v-cycle preconditioner for A.
2. Alternatively, given a right-hand-side $n-$vector $b$, HSL_MI20 solves the linear system of equations $Ax=b$ by an AMG method with or without a Krylov accelerator.

A classical AMG method is used, as described in [1] (see also Section 5 below for a brief description of the algorithm). The matrix $A$ must have positive diagonal entries and (most of) the off-diagonal entries must be negative (the diagonal should be large compared to the sum of the off-diagonals). During the multigrid coarsening process, positive off-diagonal entries are ignored and, when calculating the interpolation weights, positive off-diagonal entries are added to the diagonal.

Precision: At least 8-byte arithmetic is recommended.