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<H2><A NAME="SECTION04752000000000000000">Computational Failures and INFO > 0</A></H2>
<P>
<A NAME="6311"> </A><A NAME="6312"> </A>
A positive value of INFO on return from a ScaLAPACK routine indicates a
failure in the course of the algorithm. Common causes are
<UL>
<LI> a matrix is singular (to working precision),
<LI> a symmetric matrix is not positive definite, or
<LI> an iterative algorithm for computing eigenvalues or eigenvectors
fails to converge in the permitted number of iterations.
</UL>
For example, if PSGESVX<A NAME="6315"> </A> is called to solve a system of
equations with a coefficient matrix<A NAME="6316"> </A> that is
approximately singular,
it may detect exact singularity at the <I>i</I>th stage of the <I>LU</I>
factorization, in which case it returns INFO = <I>i</I>;
or (more probably) it may compute an estimate of the reciprocal condition number
that is less than relative machine precision, in which case it returns INFO = <I>n</I>+1.
Again, the documentation in Part <A HREF="node187.html#partroutines">ii</A> should be consulted for a
description of the error.
<P>
When a failure with INFO > 0 occurs, control is <EM>always</EM> returned
to the calling program; <TT>PXERBLA()</TT> is <EM>not</EM> called, and no error
message is written.
Thus, it is always good practice to check for
a nonzero value of INFO on return from a ScaLAPACK routine.
<P>
A failure with INFO > 0 may indicate any of the following:
<P>
<UL>
<LI> An inappropriate routine was used:.
For example, if a routine fails because a symmetric matrix turns out not to be
positive definite, consider using a routine for symmetric indefinite matrices.
<LI> A single-precision routine was used when double precision was needed.
For example, if PSGESVX<A NAME="6322"> </A> reports approximate singularity
(as illustrated above), the corresponding double precision routine PDGESVX
may be able to solve the problem (but nevertheless the problem is
ill-conditioned).
<LI> A programming error occurred in generating the data supplied
to a routine. For example, even though theoretically a matrix should be
well-conditioned and positive-definite, a programming error in generating
the matrix could easily destroy either of those properties.
<LI> A programming error occurred in calling the routine, of the kind
listed in section <A HREF="node149.html#seccommonerrors">7.3</A>.
<P>
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<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
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