NAG FL Interface
f07htf (zpbequ)

1 Purpose

f07htf computes a diagonal scaling matrix S intended to equilibrate a complex n by n Hermitian positive definite band matrix A , with bandwidth 2kd+1 , and reduce its condition number.

2 Specification

Fortran Interface
Subroutine f07htf ( uplo, n, kd, ab, ldab, s, scond, amax, info)
Integer, Intent (In) :: n, kd, ldab
Integer, Intent (Out) :: info
Real (Kind=nag_wp), Intent (Out) :: s(n), scond, amax
Complex (Kind=nag_wp), Intent (In) :: ab(ldab,*)
Character (1), Intent (In) :: uplo
C Header Interface
#include <nag.h>
void  f07htf_ (const char *uplo, const Integer *n, const Integer *kd, const Complex ab[], const Integer *ldab, double s[], double *scond, double *amax, Integer *info, const Charlen length_uplo)
The routine may be called by the names f07htf, nagf_lapacklin_zpbequ or its LAPACK name zpbequ.

3 Description

f07htf computes a diagonal scaling matrix S chosen so that
sj=1 / ajj .  
This means that the matrix B given by
B=SAS ,  
has diagonal elements equal to unity. This in turn means that the condition number of B , κ2B , is within a factor n of the matrix of smallest possible condition number over all possible choices of diagonal scalings (see Corollary 7.6 of Higham (2002)).

4 References

Higham N J (2002) Accuracy and Stability of Numerical Algorithms (2nd Edition) SIAM, Philadelphia

5 Arguments

1: uplo Character(1) Input
On entry: indicates whether the upper or lower triangular part of A is stored in the array ab, as follows:
The upper triangle of A is stored.
The lower triangle of A is stored.
Constraint: uplo='U' or 'L'.
2: n Integer Input
On entry: n, the order of the matrix A.
Constraint: n0.
3: kd Integer Input
On entry: kd, the number of superdiagonals of the matrix A if uplo='U', or the number of subdiagonals if uplo='L'.
Constraint: kd0.
4: abldab* Complex (Kind=nag_wp) array Input
Note: the second dimension of the array ab must be at least max1,n.
On entry: the upper or lower triangle of the Hermitian positive definite band matrix A whose scaling factors are to be computed.
The matrix is stored in rows 1 to kd+1, more precisely,
  • if uplo='U', the elements of the upper triangle of A within the band must be stored with element Aij in abkd+1+i-jj​ for ​max1,j-kdij;
  • if uplo='L', the elements of the lower triangle of A within the band must be stored with element Aij in ab1+i-jj​ for ​jiminn,j+kd.
Only the elements of the array ab corresponding to the diagonal elements of A are referenced. (Row kd+1 of ab when uplo='U', row 1 of ab when uplo='L'.)
5: ldab Integer Input
On entry: the first dimension of the array ab as declared in the (sub)program from which f07htf is called.
Constraint: ldabkd+1.
6: sn Real (Kind=nag_wp) array Output
On exit: if info=0, s contains the diagonal elements of the scaling matrix S.
7: scond Real (Kind=nag_wp) Output
On exit: if info=0, scond contains the ratio of the smallest value of s to the largest value of s. If scond0.1 and amax is neither too large nor too small, it is not worth scaling by S.
8: amax Real (Kind=nag_wp) Output
On exit: maxaij. If amax is very close to overflow or underflow, the matrix A should be scaled.
9: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.
The valueth diagonal element of A is not positive (and hence A cannot be positive definite).

7 Accuracy

The computed scale factors will be close to the exact scale factors.

8 Parallelism and Performance

f07htf is not threaded in any implementation.

9 Further Comments

The real analogue of this routine is f07hff.

10 Example

This example equilibrates the Hermitian positive definite matrix A given by
A = 9.39 -i1.08-1.73i -i0 -i0 1.08+1.73i -i1.69 -0.04+0.29i×1010 -i0 0 -0.04-0.29i×1010 2.65×1020 -0.33+2.24i×1010 0 -i0 -0.33-2.24i×1010 -i2.17 .  
Details of the scaling factors and the scaled matrix are output.

10.1 Program Text

Program Text (f07htfe.f90)

10.2 Program Data

Program Data (f07htfe.d)

10.3 Program Results

Program Results (f07htfe.r)