CGBEQU(l) ) CGBEQU(l)
CGBEQU - compute row and column scalings intended to equilibrate an M-by-N band matrix A
and reduce its condition number
SUBROUTINE CGBEQU( M, N, KL, KU, AB, LDAB, R, C, ROWCND, COLCND, AMAX, INFO )
INTEGER INFO, KL, KU, LDAB, M, N
REAL AMAX, COLCND, ROWCND
REAL C( * ), R( * )
COMPLEX AB( LDAB, * )
CGBEQU computes row and column scalings intended to equilibrate an M-by-N band matrix A
and reduce its condition number. R returns the row scale factors and C the column scale
factors, chosen to try to make the largest element in each row and column of the matrix B
with elements B(i,j)=R(i)*A(i,j)*C(j) have absolute value 1.
R(i) and C(j) are restricted to be between SMLNUM = smallest safe number and BIGNUM =
largest safe number. Use of these scaling factors is not guaranteed to reduce the condi-
tion number of A but works well in practice.
M (input) INTEGER
The number of rows of the matrix A. M >= 0.
N (input) INTEGER
The number of columns 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.
AB (input) COMPLEX array, dimension (LDAB,N)
The band matrix A, stored 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-
LDAB (input) INTEGER
The leading dimension of the array AB. LDAB >= KL+KU+1.
R (output) REAL array, dimension (M)
If INFO = 0, or INFO > M, R contains the row scale factors for A.
C (output) REAL array, dimension (N)
If INFO = 0, C contains the column scale factors for A.
ROWCND (output) REAL
If INFO = 0 or INFO > M, ROWCND contains the ratio of the smallest R(i) to the
largest R(i). If ROWCND >= 0.1 and AMAX is neither too large nor too small, it is
not worth scaling by R.
COLCND (output) REAL
If INFO = 0, COLCND contains the ratio of the smallest C(i) to the largest C(i).
If COLCND >= 0.1, it is not worth scaling by C.
AMAX (output) REAL
Absolute value of largest matrix element. If AMAX is very close to overflow or
very close to underflow, the matrix should be scaled.
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, and i is
<= M: the i-th row of A is exactly zero
> M: the (i-M)-th column of A is exactly zero
LAPACK version 3.0 15 June 2000 CGBEQU(l)