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HSL_MI02 Symmetric possibly-indefinite system: SYMMBK method

This routine uses the SYMMBK method to solve the n × n symmetric but possibly indefinite linear system Ax = b, optionally using preconditioning. If PPT is the preconditioning matrix, the routine actually solves the preconditioned system

Āx̄ = b̄,

with Ā = PAPT and b̄ = Pb and recovers the solution x = PTx̄. Reverse communication is used for preconditioning operations and matrix-vector products of the form Az.