\section{Preconditioner routines}
\label{sec:precs}

% \section{Preconditioners}
\label{sec:psprecs}
PSBLAS contains the implementation of many preconditioning
techniques some of which are very flexible thanks to the presence of
many parameters that is possible to adjust to fit the user's needs:
\begin{itemize}
\item Diagonal Scaling
\item Block Jacobi with ILU(0) factorization
\item Additive Schwarz with the Restricted Additive Schwarz and
  Additive Schwarz with Harmonic extensions;
\item Two-Level Additive Schwarz; this is actually a family of
  preconditioners since there is the possibility to choose between
  many variants.
\end{itemize}



\subroutine{psb\_precset}{Sets the precodntioner type}

\syntax{call psb\_precset}{prec, ptype, iv, rs, rv, ierr}

\begin{description}
\item[\bf On Entry]
\item[prec] 
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: e pronditioner data structure \precdata.
\item[ptype] the type of preconditioner. 
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a string.
\item[iv] integer parameters for the precondtioner. 
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: an integer array.
\item[rs] 
Scope: {\bf } \\
Type: {\bf }\\
Specified as: .
\item[rv] 
Scope: {\bf } \\
Type: {\bf }\\
Specified as: .
\item[ierr] 
Scope: {\bf } \\
Type: {\bf }\\
Specified as: .
\end{description}




\subroutine{psb\_precbld}{Builds a preconditioner}

\syntax{call psb\_precbld}{a, desc\_a, prec, info, upd}

\begin{description}
\item[\bf On Entry]
\item[a] the system sparse matrix.
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a sparse matrix data structure \spdata.
\item[desc\_a] the problem communication descriptor. 
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a communication descriptor data structure \descdata.
\item[upd] 
Scope: {\bf global} \\
Type: {\bf optional}\\
Specified as: a character.
\end{description}

\begin{description}
\item[\bf On Return]
\item[prec] the precodntioner.\\
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a precondtioner data structure \precdata\\
\item[info] the return error code.\\
Scope: {\bf local} \\
Type: {\bf required}\\
Specified as: an integer.\\
\end{description}



\subroutine{psb\_precaply}{Preconditioner application routine}

\syntax{call psb\_precaply}{prec,x,y,desc\_a,info,trans,work}
\syntax{call psb\_precaply}{prec,x,desc\_a,info,trans}

\begin{description}
\item[\bf On Entry]
\item[prec] the preconditioner.
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a preconditioner data structure \precdata.
\item[x] the source vector.
Scope: {\bf global} \\
Type: {\bf require}\\
Specified as: a double precision array.
\item[desc\_a] the problem communication descriptor.
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a communication data structure \descdata.
\item[trans] 
Scope: {\bf } \\
Type: {\bf optional}\\
Specified as: a character.
\item[work] an optional work space
Scope: {\bf local} \\
Type: {\bf optional}\\
Specified as: a double precision array.
\end{description}

\begin{description}
\item[\bf On Return]
\item[y] the destination vector.
Scope: {\bf global} \\
Type: {\bf required}\\
Specified as: a double precision array.
\item[info] the return error code.\\
Scope: {\bf local} \\
Type: {\bf required}\\
Specified as: an integer.\\
\end{description}



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