REAL FUNCTION SLA_GBRPVGRW( N, KL, KU, NCOLS, AB, LDAB, AFB,
$ LDAFB )
*
* -- LAPACK routine (version 3.2.2) --
* -- Contributed by James Demmel, Deaglan Halligan, Yozo Hida and --
* -- Jason Riedy of Univ. of California Berkeley. --
* -- June 2010 --
*
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley and NAG Ltd. --
*
IMPLICIT NONE
* ..
* .. Scalar Arguments ..
INTEGER N, KL, KU, NCOLS, LDAB, LDAFB
* ..
* .. Array Arguments ..
REAL AB( LDAB, * ), AFB( LDAFB, * )
* ..
*
* Purpose
* =======
*
* SLA_GBRPVGRW computes the reciprocal pivot growth factor
* norm(A)/norm(U). The "max absolute element" norm is used. If this is
* much less than 1, the stability of the LU factorization of the
* (equilibrated) matrix A could be poor. This also means that the
* solution X, estimated condition numbers, and error bounds could be
* unreliable.
*
* Arguments
* =========
*
* N (input) INTEGER
* The number of linear equations, i.e., the order of the
* matrix A. N >= 0.
*
* KL (input) INTEGER
* The number of subdiagonals within the band of A. KL >= 0.
*
* KU (input) INTEGER
* The number of superdiagonals within the band of A. KU >= 0.
*
* NCOLS (input) INTEGER
* The number of columns of the matrix A. NCOLS >= 0.
*
* AB (input) REAL array, dimension (LDAB,N)
* On entry, the matrix A in band storage, in rows 1 to KL+KU+1.
* The j-th column of A is stored in the j-th column of the
* array AB as follows:
* AB(KU+1+i-j,j) = A(i,j) for max(1,j-KU)<=i<=min(N,j+kl)
*
* LDAB (input) INTEGER
* The leading dimension of the array AB. LDAB >= KL+KU+1.
*
* AFB (input) REAL array, dimension (LDAFB,N)
* Details of the LU factorization of the band matrix A, as
* computed by SGBTRF. U is stored as an upper triangular
* band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1,
* and the multipliers used during the factorization are stored
* in rows KL+KU+2 to 2*KL+KU+1.
*
* LDAFB (input) INTEGER
* The leading dimension of the array AFB. LDAFB >= 2*KL+KU+1.
*
* =====================================================================
*
* .. Local Scalars ..
INTEGER I, J, KD
REAL AMAX, UMAX, RPVGRW
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, MAX, MIN
* ..
* .. Executable Statements ..
*
RPVGRW = 1.0
KD = KU + 1
DO J = 1, NCOLS
AMAX = 0.0
UMAX = 0.0
DO I = MAX( J-KU, 1 ), MIN( J+KL, N )
AMAX = MAX( ABS( AB( KD+I-J, J)), AMAX )
END DO
DO I = MAX( J-KU, 1 ), J
UMAX = MAX( ABS( AFB( KD+I-J, J ) ), UMAX )
END DO
IF ( UMAX /= 0.0 ) THEN
RPVGRW = MIN( AMAX / UMAX, RPVGRW )
END IF
END DO
SLA_GBRPVGRW = RPVGRW
END