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Preconditioner data structure

Our base library offers support for simple well known preconditioners like Diagonal Scaling or Block Jacobi with incomplete factorization ILU(0).

A preconditioner is held in the precdata`psb_prec_type` data structure reported in figure 5. The `psb_prec_type` data type may contain a simple preconditioning matrix with the associated communication descriptor.The values contained in the `iprcparm` and `rprcparm` define tha type of preconditioner along with all the parameters related to it; thus, `iprcparm` and `rprcparm` define how the other records have to be interpreted. This data structure is the basis of more complex preconditioning strategies, which are the subject of further research.

Figure 5: The PSBLAS defined data type that contains a preconditioner.