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\section*{Abstract}
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\addcontentsline{toc}{section}{Abstract}
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\textsc{MLD2P4 (MultiLevel Domain Decomposition Parallel Preconditioners Package
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based on PSBLAS}) is a package of parallel algebraic multilevel preconditioners.
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The first release of MLD2P4 made available multilevel additive and hybrid Schwarz
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preconditioners, as well as one-level additive Schwarz preconditioners. The package
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has been extended to include further multilevel cycles and smoothers widely used in
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multigrid methods. In the multilevel case, a purely algebraic approach is applied to
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generate coarse-level corrections, so that no geometric background is needed
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concerning the matrix to be preconditioned. The matrix is assumed to be square,
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real or complex.
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MLD2P4 has been designed to provide scalable and easy-to-use preconditioners
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in the context of the PSBLAS (Parallel Sparse Basic Linear Algebra Subprograms)
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computational framework and can be used in conjuction with the Krylov solvers
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available in this framework. MLD2P4 enables the user to easily specify different
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features of an algebraic multilevel preconditioner, thus allowing to search
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for the ``best'' preconditioner for the problem at hand.
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The package employs object-oriented design techniques in
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Fortran~2003, with interfaces to additional third party libraries
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such as MUMPS, UMFPACK, SuperLU, and SuperLU\_Dist, which
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can be exploited in building multilevel preconditioners. The parallel
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implementation is based on a Single Program Multiple Data (SPMD)
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paradigm; the inter-process communication is based on MPI and
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is managed mainly through PSBLAS.
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This guide provides a brief description of the functionalities and
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the user interface of MLD2P4.
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