```      SUBROUTINE DTBCON( NORM, UPLO, DIAG, N, KD, AB, LDAB, RCOND, WORK,
\$                   IWORK, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     Modified to call DLACN2 in place of DLACON, 5 Feb 03, SJH.
*
*     .. Scalar Arguments ..
CHARACTER          DIAG, NORM, UPLO
INTEGER            INFO, KD, LDAB, N
DOUBLE PRECISION   RCOND
*     ..
*     .. Array Arguments ..
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 1-norm or the infinity-norm.
*
*  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 1-norm condition number or the
*          infinity-norm condition number is required:
*          = '1' or 'O':  1-norm;
*          = 'I':         Infinity-norm.
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  A is upper triangular;
*          = 'L':  A is lower triangular.
*
*  DIAG    (input) CHARACTER*1
*          = 'N':  A is non-unit 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 j-th column of A is stored
*          in the j-th column of the array AB as follows:
*          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
*          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
*          If DIAG = 'U', the diagonal elements 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 i-th argument had an illegal value
*
*  =====================================================================
*
*     .. Parameters ..
DOUBLE PRECISION   ONE, ZERO
PARAMETER          ( ONE = 1.0D+0, ZERO = 0.0D+0 )
*     ..
*     .. Local Scalars ..
LOGICAL            NOUNIT, ONENRM, UPPER
CHARACTER          NORMIN
INTEGER            IX, KASE, KASE1
DOUBLE PRECISION   AINVNM, ANORM, SCALE, SMLNUM, XNORM
*     ..
*     .. Local Arrays ..
INTEGER            ISAVE( 3 )
*     ..
*     .. External Functions ..
LOGICAL            LSAME
INTEGER            IDAMAX
DOUBLE PRECISION   DLAMCH, DLANTB
EXTERNAL           LSAME, IDAMAX, DLAMCH, DLANTB
*     ..
*     .. External Subroutines ..
EXTERNAL           DLACN2, DLATBS, DRSCL, XERBLA
*     ..
*     .. Intrinsic Functions ..
INTRINSIC          ABS, DBLE, MAX
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
INFO = 0
UPPER = LSAME( UPLO, 'U' )
ONENRM = NORM.EQ.'1' .OR. LSAME( NORM, 'O' )
NOUNIT = LSAME( DIAG, 'N' )
*
IF( .NOT.ONENRM .AND. .NOT.LSAME( NORM, 'I' ) ) THEN
INFO = -1
ELSE IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
INFO = -2
ELSE IF( .NOT.NOUNIT .AND. .NOT.LSAME( DIAG, 'U' ) ) THEN
INFO = -3
ELSE IF( N.LT.0 ) THEN
INFO = -4
ELSE IF( KD.LT.0 ) THEN
INFO = -5
ELSE IF( LDAB.LT.KD+1 ) THEN
INFO = -7
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'DTBCON', -INFO )
RETURN
END IF
*
*     Quick return if possible
*
IF( N.EQ.0 ) THEN
RCOND = ONE
RETURN
END IF
*
RCOND = ZERO
SMLNUM = DLAMCH( 'Safe minimum' )*DBLE( MAX( 1, N ) )
*
*     Compute the norm of the triangular matrix A.
*
ANORM = DLANTB( NORM, UPLO, DIAG, N, KD, AB, LDAB, WORK )
*
*     Continue only if ANORM > 0.
*
IF( ANORM.GT.ZERO ) THEN
*
*        Estimate the norm of the inverse of A.
*
AINVNM = ZERO
NORMIN = 'N'
IF( ONENRM ) THEN
KASE1 = 1
ELSE
KASE1 = 2
END IF
KASE = 0
10    CONTINUE
CALL DLACN2( N, WORK( N+1 ), WORK, IWORK, AINVNM, KASE, ISAVE )
IF( KASE.NE.0 ) THEN
IF( KASE.EQ.KASE1 ) THEN
*
*              Multiply by inv(A).
*
CALL DLATBS( UPLO, 'No transpose', DIAG, NORMIN, N, KD,
\$                      AB, LDAB, WORK, SCALE, WORK( 2*N+1 ), INFO )
ELSE
*
*              Multiply by inv(A').
*
CALL DLATBS( UPLO, 'Transpose', DIAG, NORMIN, N, KD, AB,
\$                      LDAB, WORK, SCALE, WORK( 2*N+1 ), INFO )
END IF
NORMIN = 'Y'
*
*           Multiply by 1/SCALE if doing so will not cause overflow.
*
IF( SCALE.NE.ONE ) THEN
IX = IDAMAX( N, WORK, 1 )
XNORM = ABS( WORK( IX ) )
IF( SCALE.LT.XNORM*SMLNUM .OR. SCALE.EQ.ZERO )
\$            GO TO 20
CALL DRSCL( N, SCALE, WORK, 1 )
END IF
GO TO 10
END IF
*
*        Compute the estimate of the reciprocal condition number.
*
IF( AINVNM.NE.ZERO )
\$      RCOND = ( ONE / ANORM ) / AINVNM
END IF
*
20 CONTINUE
RETURN
*
*     End of DTBCON
*
END

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