NAG Library Function Document

nag_zhb_norm (f16uec)

void	nag_zhb_norm (Nag_OrderType order, Nag_NormType norm, Nag_UploType uplo, Integer n, Integer k, const Complex ab[], Integer pdab, double r, NagError fail)

3 Description

Given a complex

n

n

Hermitian band matrix,

A

, nag_zhb_norm (f16uec) calculates one of the values given by

{‖A‖}_{1} = \max_{j} \sum_{i = 1}^{n} |a_{i j}|,

{‖A‖}_{\infty} = \max_{i} \sum_{j = 1}^{n} |a_{i j}|,

{‖A‖}_{F} = {(\sum_{i = 1}^{n} \sum_{j = 1}^{n} {|a_{i j}|}^{2})}^{1 / 2}

\max_{i, j} |a_{i j}| .

Note that, since

A

is symmetric,

{‖A‖}_{1} = {‖A‖}_{\infty}

4 References

Basic Linear Algebra Subprograms Technical (BLAST) Forum (2001) Basic Linear Algebra Subprograms Technical (BLAST) Forum Standard University of Tennessee, Knoxville, Tennessee http://www.netlib.org/blas/blast-forum/blas-report.pdf

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 3.2.1.3 in the Essential Introduction for a more detailed explanation of the use of this argument.

Constraint:

order = Nag_RowMajor

Nag_ColMajor

2: norm – Nag_NormTypeInput

On entry: specifies the value to be returned.

$norm = Nag_OneNorm$: The $1$ -norm.
$norm = Nag_InfNorm$: The $\infty$ -norm.
$norm = Nag_FrobeniusNorm$: The Frobenius (or Euclidean) norm.
$norm = Nag_MaxNorm$: The value $\max_{i, j} |a_{i j}|$ (not a norm).

Constraint:

norm = Nag_OneNorm

Nag_InfNorm

Nag_FrobeniusNorm

Nag_MaxNorm

3: uplo – Nag_UploTypeInput

On entry: specifies whether the upper or lower triangular part of

A

is stored.

$uplo = Nag_Upper$: The upper triangular part of $A$ is stored.
$uplo = Nag_Lower$: The lower triangular part of $A$ is stored.

Constraint:

uplo = Nag_Upper

Nag_Lower

4: n – IntegerInput

On entry:

n

, the order of the matrix

A

n = 0

, then n is set to zero.

Constraint:

n \geq 0

5: k – IntegerInput

On entry:

k

, the number of subdiagonals or superdiagonals of the matrix

A

Constraint:

k \geq 0

6: ab[ $\dim$ ] – const ComplexInput

Note: the dimension, dim, of the array ab must be at least

\max (1, pdab \times n)

On entry: the

n

n

Hermitian band matrix

A

This is stored as a notional two-dimensional array with row elements or column elements stored contiguously. The storage of elements of

A_{i j}

, depends on the order and uplo arguments as follows:

if $order = Nag_ColMajor$ and $uplo = Nag_Upper$ ,
$A_{i j}$ is stored in $ab [k + i - j + (j - 1) \times pdab]$ , for $j = 1, \dots, n$ and $i = \max (1, j - k), \dots, j$ ;
if $order = Nag_ColMajor$ and $uplo = Nag_Lower$ ,
$A_{i j}$ is stored in $ab [i - j + (j - 1) \times pdab]$ , for $j = 1, \dots, n$ and $i = j, \dots, \min (n, j + k)$ ;
if $order = Nag_RowMajor$ and $uplo = Nag_Upper$ ,
$A_{i j}$ is stored in $ab [j - i + (i - 1) \times pdab]$ , for $i = 1, \dots, n$ and $j = i, \dots, \min (n, i + k)$ ;
if $order = Nag_RowMajor$ and $uplo = Nag_Lower$ ,
$A_{i j}$ is stored in $ab [k + j - i + (i - 1) \times pdab]$ , for $i = 1, \dots, n$ and $j = \max (1, i - k), \dots, i$ .

7: pdab – IntegerInput

On entry: the stride separating row or column elements (depending on the value of order) of the matrix

A

in the array ab.

Constraint:

pdab \geq k + 1

8: r – double *Output

On exit: the value of the norm specified by norm.

9: fail – NagError *Input/Output

The NAG error argument (see Section 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_ALLOC_FAIL: Dynamic memory allocation failed.
NE_BAD_PARAM: On entry, argument $⟨value⟩$ had an illegal value.
NE_INT: On entry, $k = ⟨value⟩$ .
Constraint: $k \geq 0$ .
On entry, $n = ⟨value⟩$ .
Constraint: $n \geq 0$ .
NE_INT_2: On entry, $pdab = ⟨value⟩$ , $k = ⟨value⟩$ .
Constraint: $pdab \geq k + 1$ .

7 Accuracy

The BLAS standard requires accurate implementations which avoid unnecessary over/underflow (see Section 2.7 of Basic Linear Algebra Subprograms Technical (BLAST) Forum (2001)).