void	f16rec (Nag_OrderType order, Nag_NormType norm, Nag_UploType uplo, Integer n, Integer k, const double ab[], Integer pdab, double r, NagError fail)

The function may be called by the names: f16rec, nag_blast_dsb_norm or nag_dsb_norm.

3 Description

Given a real

n

n

symmetric band matrix,

A

, f16rec 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 https://www.netlib.org/blas/blast-forum/blas-report.pdf

5 Arguments

1: $order$ – Nag_OrderType Input

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 3.1.3 in the Introduction to the NAG Library CL Interface for a more detailed explanation of the use of this argument.

Constraint:

order = Nag_RowMajor

Nag_ColMajor

2: $norm$ – Nag_NormType Input

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_UploType Input

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: $n$ – Integer Input

On entry:

n

, the order of the matrix

A

n = 0

, n is set to zero.

Constraint:

n \geq 0

5: $k$ – Integer Input

On entry:

k

, the number of subdiagonals or superdiagonals of the matrix

A

Constraint:

k \geq 0

6: $ab [\dim]$ – const double Input

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

\max (1, pdab \times n)

On entry: the

n

n

symmetric 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: $pdab$ – Integer Input

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 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

NE_ALLOC_FAIL: Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
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: $pdab \geq k + 1$ .
NE_NO_LICENCE: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.

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)).