.TH "TESTING/EIG/dstech.f" 3 "Version 3.12.0" "LAPACK" \" -*- nroff -*-
.ad l
.nh
.SH NAME
TESTING/EIG/dstech.f
.SH SYNOPSIS
.br
.PP
.SS "Functions/Subroutines"

.in +1c
.ti -1c
.RI "subroutine \fBdstech\fP (n, a, b, eig, tol, work, info)"
.br
.RI "\fBDSTECH\fP "
.in -1c
.SH "Function/Subroutine Documentation"
.PP 
.SS "subroutine dstech (integer n, double precision, dimension( * ) a, double precision, dimension( * ) b, double precision, dimension( * ) eig, double precision tol, double precision, dimension( * ) work, integer info)"

.PP
\fBDSTECH\fP 
.PP
\fBPurpose:\fP
.RS 4

.PP
.nf
    Let T be the tridiagonal matrix with diagonal entries A(1) ,\&.\&.\&.,
    A(N) and offdiagonal entries B(1) ,\&.\&.\&., B(N-1))\&.  DSTECH checks to
    see if EIG(1) ,\&.\&.\&., EIG(N) are indeed accurate eigenvalues of T\&.
    It does this by expanding each EIG(I) into an interval
    [SVD(I) - EPS, SVD(I) + EPS], merging overlapping intervals if
    any, and using Sturm sequences to count and verify whether each
    resulting interval has the correct number of eigenvalues (using
    DSTECT)\&.  Here EPS = TOL*MAZHEPS*MAXEIG, where MACHEPS is the
    machine precision and MAXEIG is the absolute value of the largest
    eigenvalue\&. If each interval contains the correct number of
    eigenvalues, INFO = 0 is returned, otherwise INFO is the index of
    the first eigenvalue in the first bad interval\&.
.fi
.PP
 
.RE
.PP
\fBParameters\fP
.RS 4
\fIN\fP 
.PP
.nf
          N is INTEGER
          The dimension of the tridiagonal matrix T\&.
.fi
.PP
.br
\fIA\fP 
.PP
.nf
          A is DOUBLE PRECISION array, dimension (N)
          The diagonal entries of the tridiagonal matrix T\&.
.fi
.PP
.br
\fIB\fP 
.PP
.nf
          B is DOUBLE PRECISION array, dimension (N-1)
          The offdiagonal entries of the tridiagonal matrix T\&.
.fi
.PP
.br
\fIEIG\fP 
.PP
.nf
          EIG is DOUBLE PRECISION array, dimension (N)
          The purported eigenvalues to be checked\&.
.fi
.PP
.br
\fITOL\fP 
.PP
.nf
          TOL is DOUBLE PRECISION
          Error tolerance for checking, a multiple of the
          machine precision\&.
.fi
.PP
.br
\fIWORK\fP 
.PP
.nf
          WORK is DOUBLE PRECISION array, dimension (N)
.fi
.PP
.br
\fIINFO\fP 
.PP
.nf
          INFO is INTEGER
          0  if the eigenvalues are all correct (to within
             1 +- TOL*MAZHEPS*MAXEIG)
          >0 if the interval containing the INFO-th eigenvalue
             contains the incorrect number of eigenvalues\&.
.fi
.PP
 
.RE
.PP
\fBAuthor\fP
.RS 4
Univ\&. of Tennessee 
.PP
Univ\&. of California Berkeley 
.PP
Univ\&. of Colorado Denver 
.PP
NAG Ltd\&. 
.RE
.PP

.PP
Definition at line \fB100\fP of file \fBdstech\&.f\fP\&.
.SH "Author"
.PP 
Generated automatically by Doxygen for LAPACK from the source code\&.