
DGTCON(l) ) DGTCON(l)
NAME
DGTCON  estimate the reciprocal of the condition number of a real tridiagonal matrix A
using the LU factorization as computed by DGTTRF
SYNOPSIS
SUBROUTINE DGTCON( NORM, N, DL, D, DU, DU2, IPIV, ANORM, RCOND, WORK, IWORK, INFO )
CHARACTER NORM
INTEGER INFO, N
DOUBLE PRECISION ANORM, RCOND
INTEGER IPIV( * ), IWORK( * )
DOUBLE PRECISION D( * ), DL( * ), DU( * ), DU2( * ), WORK( * )
PURPOSE
DGTCON estimates the reciprocal of the condition number of a real tridiagonal matrix A
using the LU factorization as computed by DGTTRF. An estimate is obtained for
norm(inv(A)), and the reciprocal of the condition number is computed as RCOND = 1 / (ANORM
* norm(inv(A))).
ARGUMENTS
NORM (input) CHARACTER*1
Specifies whether the 1norm condition number or the infinitynorm condition num
ber is required:
= '1' or 'O': 1norm;
= 'I': Infinitynorm.
N (input) INTEGER
The order of the matrix A. N >= 0.
DL (input) DOUBLE PRECISION array, dimension (N1)
The (n1) multipliers that define the matrix L from the LU factorization of A as
computed by DGTTRF.
D (input) DOUBLE PRECISION array, dimension (N)
The n diagonal elements of the upper triangular matrix U from the LU factorization
of A.
DU (input) DOUBLE PRECISION array, dimension (N1)
The (n1) elements of the first superdiagonal of U.
DU2 (input) DOUBLE PRECISION array, dimension (N2)
The (n2) elements of the second superdiagonal of U.
IPIV (input) INTEGER array, dimension (N)
The pivot indices; for 1 <= i <= n, row i of the matrix was interchanged with row
IPIV(i). IPIV(i) will always be either i or i+1; IPIV(i) = i indicates a row
interchange was not required.
ANORM (input) DOUBLE PRECISION
If NORM = '1' or 'O', the 1norm of the original matrix A. If NORM = 'I', the
infinitynorm of the original matrix A.
RCOND (output) DOUBLE PRECISION
The reciprocal of the condition number of the matrix A, computed as RCOND =
1/(ANORM * AINVNM), where AINVNM is an estimate of the 1norm of inv(A) computed
in this routine.
WORK (workspace) DOUBLE PRECISION array, dimension (2*N)
IWORK (workspace) INTEGER array, dimension (N)
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
LAPACK version 3.0 15 June 2000 DGTCON(l) 
