
DTBCON(l) ) DTBCON(l)
NAME
DTBCON  estimate the reciprocal of the condition number of a triangular band matrix A, in
either the 1norm or the infinitynorm
SYNOPSIS
SUBROUTINE DTBCON( NORM, UPLO, DIAG, N, KD, AB, LDAB, RCOND, WORK, IWORK, INFO )
CHARACTER DIAG, NORM, UPLO
INTEGER INFO, KD, LDAB, N
DOUBLE PRECISION RCOND
INTEGER IWORK( * )
DOUBLE PRECISION AB( LDAB, * ), WORK( * )
PURPOSE
DTBCON estimates the reciprocal of the condition number of a triangular band matrix A, in
either the 1norm or the infinitynorm. The norm of A is computed and an estimate is
obtained for norm(inv(A)), then the reciprocal of the condition number is computed as
RCOND = 1 / ( norm(A) * 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.
UPLO (input) CHARACTER*1
= 'U': A is upper triangular;
= 'L': A is lower triangular.
DIAG (input) CHARACTER*1
= 'N': A is nonunit triangular;
= 'U': A is unit triangular.
N (input) INTEGER
The order of the matrix A. N >= 0.
KD (input) INTEGER
The number of superdiagonals or subdiagonals of the triangular band matrix A. KD
>= 0.
AB (input) DOUBLE PRECISION array, dimension (LDAB,N)
The upper or lower triangular band matrix A, stored in the first kd+1 rows of the
array. The jth column of A is stored in the jth column of the array AB as fol
lows: if UPLO = 'U', AB(kd+1+ij,j) = A(i,j) for max(1,jkd)<=i<=j; if UPLO = 'L',
AB(1+ij,j) = A(i,j) for j<=i<=min(n,j+kd). If DIAG = 'U', the diagonal ele
ments of A are not referenced and are assumed to be 1.
LDAB (input) INTEGER
The leading dimension of the array AB. LDAB >= KD+1.
RCOND (output) DOUBLE PRECISION
The reciprocal of the condition number of the matrix A, computed as RCOND =
1/(norm(A) * norm(inv(A))).
WORK (workspace) DOUBLE PRECISION array, dimension (3*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 DTBCON(l) 
