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SUBROUTINE SGTCON( NORM, N, DL, D, DU, DU2, IPIV, ANORM, RCOND,
$ WORK, IWORK, INFO )
*
* -- LAPACK routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* September 30, 1994
*
* .. Scalar Arguments ..
CHARACTER NORM
INTEGER INFO, N
REAL ANORM, RCOND
* ..
* .. Array Arguments ..
INTEGER IPIV( * ), IWORK( * )
REAL D( * ), DL( * ), DU( * ), DU2( * ), WORK( * )
* ..
*
* Purpose
* =======
*
* SGTCON estimates the reciprocal of the condition number of a real
* tridiagonal matrix A using the LU factorization as computed by
* SGTTRF.
*
* 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 1-norm condition number or the
* infinity-norm condition number is required:
* = '1' or 'O': 1-norm;
* = 'I': Infinity-norm.
*
* N (input) INTEGER
* The order of the matrix A. N >= 0.
*
* DL (input) REAL array, dimension (N-1)
* The (n-1) multipliers that define the matrix L from the
* LU factorization of A as computed by SGTTRF.
*
* D (input) REAL array, dimension (N)
* The n diagonal elements of the upper triangular matrix U from
* the LU factorization of A.
*
* DU (input) REAL array, dimension (N-1)
* The (n-1) elements of the first superdiagonal of U.
*
* DU2 (input) REAL array, dimension (N-2)
* The (n-2) 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) REAL
* If NORM = '1' or 'O', the 1-norm of the original matrix A.
* If NORM = 'I', the infinity-norm of the original matrix A.
*
* RCOND (output) REAL
* The reciprocal of the condition number of the matrix A,
* computed as RCOND = 1/(ANORM * AINVNM), where AINVNM is an
* estimate of the 1-norm of inv(A) computed in this routine.
*
* WORK (workspace) REAL array, dimension (2*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 ..
REAL ONE, ZERO
PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 )
* ..
* .. Local Scalars ..
LOGICAL ONENRM
INTEGER I, KASE, KASE1
REAL AINVNM
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL SGTTRS, SLACON, XERBLA
* ..
* .. Executable Statements ..
*
* Test the input arguments.
*
INFO = 0
ONENRM = NORM.EQ.'1' .OR. LSAME( NORM, 'O' )
IF( .NOT.ONENRM .AND. .NOT.LSAME( NORM, 'I' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( ANORM.LT.ZERO ) THEN
INFO = -8
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SGTCON', -INFO )
RETURN
END IF
*
* Quick return if possible
*
RCOND = ZERO
IF( N.EQ.0 ) THEN
RCOND = ONE
RETURN
ELSE IF( ANORM.EQ.ZERO ) THEN
RETURN
END IF
*
* Check that D(1:N) is non-zero.
*
DO 10 I = 1, N
IF( D( I ).EQ.ZERO )
$ RETURN
10 CONTINUE
*
AINVNM = ZERO
IF( ONENRM ) THEN
KASE1 = 1
ELSE
KASE1 = 2
END IF
KASE = 0
20 CONTINUE
CALL SLACON( N, WORK( N+1 ), WORK, IWORK, AINVNM, KASE )
IF( KASE.NE.0 ) THEN
IF( KASE.EQ.KASE1 ) THEN
*
* Multiply by inv(U)*inv(L).
*
CALL SGTTRS( 'No transpose', N, 1, DL, D, DU, DU2, IPIV,
$ WORK, N, INFO )
ELSE
*
* Multiply by inv(L')*inv(U').
*
CALL SGTTRS( 'Transpose', N, 1, DL, D, DU, DU2, IPIV, WORK,
$ N, INFO )
END IF
GO TO 20
END IF
*
* Compute the estimate of the reciprocal condition number.
*
IF( AINVNM.NE.ZERO )
$ RCOND = ( ONE / AINVNM ) / ANORM
*
RETURN
*
* End of SGTCON
*
END
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