NAG Library Function Document

nag_dgbcon (f07bgc)

 Contents

    1  Purpose
    7  Accuracy

1
Purpose

nag_dgbcon (f07bgc) estimates the condition number of a real band matrix A, where A has been factorized by nag_dgbtrf (f07bdc).

2
Specification

#include <nag.h>
#include <nagf07.h>
void  nag_dgbcon (Nag_OrderType order, Nag_NormType norm, Integer n, Integer kl, Integer ku, const double ab[], Integer pdab, const Integer ipiv[], double anorm, double *rcond, NagError *fail)

3
Description

nag_dgbcon (f07bgc) estimates the condition number of a real band matrix A, in either the 1-norm or the -norm:
κ1A=A1A-11   or   κA=AA-1 .  
Note that κA=κ1AT.
Because the condition number is infinite if A is singular, the function actually returns an estimate of the reciprocal of the condition number.
The function should be preceded by a call to nag_dgb_norm (f16rbc) to compute A1 or A, and a call to nag_dgbtrf (f07bdc) to compute the LU factorization of A. The function then uses Higham's implementation of Hager's method (see Higham (1988)) to estimate A-11 or A-1.

4
References

Higham N J (1988) FORTRAN codes for estimating the one-norm of a real or complex matrix, with applications to condition estimation ACM Trans. Math. Software 14 381–396

5
Arguments

1:     order Nag_OrderTypeInput
On entry: the order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by order=Nag_RowMajor. See Section 3.3.1.3 in How to Use the NAG Library and its Documentation for a more detailed explanation of the use of this argument.
Constraint: order=Nag_RowMajor or Nag_ColMajor.
2:     norm Nag_NormTypeInput
On entry: indicates whether κ1A or κA is estimated.
norm=Nag_OneNorm
κ1A is estimated.
norm=Nag_InfNorm
κA is estimated.
Constraint: norm=Nag_OneNorm or Nag_InfNorm.
3:     n IntegerInput
On entry: n, the order of the matrix A.
Constraint: n0.
4:     kl IntegerInput
On entry: kl, the number of subdiagonals within the band of the matrix A.
Constraint: kl0.
5:     ku IntegerInput
On entry: ku, the number of superdiagonals within the band of the matrix A.
Constraint: ku0.
6:     ab[dim] const doubleInput
Note: the dimension, dim, of the array ab must be at least max1,pdab×n.
On entry: the LU factorization of A, as returned by nag_dgbtrf (f07bdc).
7:     pdab IntegerInput
On entry: the stride separating row or column elements (depending on the value of order) of the matrix in the array ab.
Constraint: pdab2×kl+ku+1.
8:     ipiv[dim] const IntegerInput
Note: the dimension, dim, of the array ipiv must be at least max1,n.
On entry: the pivot indices, as returned by nag_dgbtrf (f07bdc).
9:     anorm doubleInput
On entry: if norm=Nag_OneNorm, the 1-norm of the original matrix A.
If norm=Nag_InfNorm, the -norm of the original matrix A.
anorm may be computed by calling nag_dgb_norm (f16rbc) with the same value for the argument norm.
anorm must be computed either before calling nag_dgbtrf (f07bdc) or else from a copy of the original matrix A (see Section 10).
Constraint: anorm0.0.
10:   rcond double *Output
On exit: an estimate of the reciprocal of the condition number of A. rcond is set to zero if exact singularity is detected or the estimate underflows. If rcond is less than machine precision, A is singular to working precision.
11:   fail NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

6
Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
NE_BAD_PARAM
On entry, argument value had an illegal value.
NE_INT
On entry, kl=value.
Constraint: kl0.
On entry, ku=value.
Constraint: ku0.
On entry, n=value.
Constraint: n0.
On entry, pdab=value.
Constraint: pdab>0.
NE_INT_3
On entry, pdab=value, kl=value and ku=value.
Constraint: pdab2×kl+ku+1.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
NE_REAL
On entry, anorm=value.
Constraint: anorm0.0.

7
Accuracy

The computed estimate rcond is never less than the true value ρ, and in practice is nearly always less than 10ρ, although examples can be constructed where rcond is much larger.

8
Parallelism and Performance

nag_dgbcon (f07bgc) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the x06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9
Further Comments

A call to nag_dgbcon (f07bgc) involves solving a number of systems of linear equations of the form Ax=b or ATx=b; the number is usually 4 or 5 and never more than 11. Each solution involves approximately 2n2kl+ku floating-point operations (assuming nkl and nku) but takes considerably longer than a call to nag_dgbtrs (f07bec) with one right-hand side, because extra care is taken to avoid overflow when A is approximately singular.
The complex analogue of this function is nag_zgbcon (f07buc).

10
Example

This example estimates the condition number in the 1-norm of the matrix A, where
A= -0.23 2.54 -3.66 0.00 -6.98 2.46 -2.73 -2.13 0.00 2.56 2.46 4.07 0.00 0.00 -4.78 -3.82 .  
Here A is nonsymmetric and is treated as a band matrix, which must first be factorized by nag_dgbtrf (f07bdc). The true condition number in the 1-norm is 56.40.

10.1
Program Text

Program Text (f07bgce.c)

10.2
Program Data

Program Data (f07bgce.d)

10.3
Program Results

Program Results (f07bgce.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017