Integer, Intent (In)	::	m, n, lda
Integer, Intent (Inout)	::	ifail
Complex (Kind=nag_wp), Intent (Inout)	::	a(lda,*)
Complex (Kind=nag_wp), Intent (Out)	::	theta(m)

C Header Interface

#include <nag.h>

void	f01rgf_ (const Integer m, const Integer n, Complex a[], const Integer lda, Complex theta[], Integer ifail)

The routine may be called by the names f01rgf or nagf_matop_complex_trapez_rq.

3 Description

The

m \times n (m \leq n)

upper trapezoidal matrix

A

given by

A = (\begin{matrix} U & X \end{matrix}),

where

U

is an

m \times m

upper triangular matrix, is factorized as

A = (\begin{matrix} R & 0 \end{matrix}) P^{H},

where

P

is an

n \times n

unitary matrix and

R

is an

m \times m

upper triangular matrix.

P

is given as a sequence of Householder transformation matrices

P = P_{m} \dots P_{2} P_{1},

the

(m - k + 1)

th transformation matrix,

P_{k}

, being used to introduce zeros into the

k

th row of

A

P_{k}

has the form

P_{k} = (\begin{array}{l} I & 0 \\ 0 & T_{k} \end{array}),

where

\begin{matrix} T_{k} = I - γ_{k} u_{k} u_{k}^{H}, \\ u_{k} = (\begin{matrix} ζ_{k} \\ 0 \\ z_{k} \\ c r \end{matrix}), \end{matrix}

γ_{k}

is a scalar for which

Re (γ_{k}) = 1.0

ζ_{k}

is a real scalar and

z_{k}

is an

(n - m)

element vector.

γ_{k}

ζ_{k}

and

z_{k}

are chosen to annihilate the elements of the

k

th row of

X

and to make the diagonal elements of

R

real.

The scalar

γ_{k}

and the vector

u_{k}

are returned in the

k

th element of the array theta and in the

k

th row of a, such that

θ_{k}

, given by

θ_{k} = (ζ_{k}, Im (γ_{k})),

is in

theta (k)

and the elements of

z_{k}

are in

a (k, m + 1), \dots, a (k, n)

. The elements of

R

are returned in the upper triangular part of a.

For further information on this factorization and its use see Section 6.5 of Golub and Van Loan (1996).

4 References

Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

Wilkinson J H (1965) The Algebraic Eigenvalue Problem Oxford University Press, Oxford

5 Arguments

1: $m$ – Integer Input

On entry:

m

, the number of rows of the matrix

A

When

m = 0

then an immediate return is effected.

Constraint:

m \geq 0

2: $n$ – Integer Input

On entry:

n

, the number of columns of the matrix

A

Constraint:

n \geq m

3: $a (lda, *)$ – Complex (Kind=nag_wp) array Input/Output

Note: the second dimension of the array a must be at least

\max (1, n)

On entry: the leading

m \times n

upper trapezoidal part of the array a must contain the matrix to be factorized.

On exit: the

m \times m

upper triangular part of a will contain the upper triangular matrix

R

, and the

m \times (n - m)

upper trapezoidal part of a will contain details of the factorization as described in Section 3.

4: $lda$ – Integer Input

On entry: the first dimension of the array a as declared in the (sub)program from which f01rgf is called.

Constraint:

lda \geq \max (1, m)

5: $theta (m)$ – Complex (Kind=nag_wp) array Output

On exit:

theta (k)

contains the scalar

θ_{k}

for the

(m - k + 1)

th transformation. If

T_{k} = I

then

theta (k) = 0.0

; if

T_{k} = (\begin{array}{l} α & 0 \\ 0 & I \end{array}), Re (α) < 0.0

then

theta (k) = α

, otherwise

theta (k)

contains

θ_{k}

as described in Section 3 and

Re (θ_{k})

is always in the range

(1.0, \sqrt{2.0})

6: $ifail$ – Integer Input/Output

On entry: ifail must be set to

0

−1

1

to set behaviour on detection of an error; these values have no effect when no error is detected.

A value of

0

causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of

−1

means that an error message is printed while a value of

1

means that it is not.

If halting is not appropriate, the value

−1

1

is recommended. If message printing is undesirable, then the value

1

is recommended. Otherwise, the value

0

is recommended. When the value $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit:

ifail = 0

unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = - 1$: On entry, $lda = ⟨ value ⟩$ and $m = ⟨ value ⟩$ .
Constraint: $lda \geq m$ .

On entry, $m = ⟨ value ⟩$ .
Constraint: $m \geq 0$ .

On entry, $n = ⟨ value ⟩$ and $m = ⟨ value ⟩$ .
Constraint: $n \geq m$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The computed factors

R

and

P

satisfy the relation

(\begin{matrix} R & 0 \end{matrix}) P^{H} = A + E,

where

‖ E ‖ \leq c ε ‖ A ‖,

ε

is the machine precision (see x02ajf),

c

is a modest function of

m

and

n

, and

‖ . ‖

denotes the spectral (two) norm.

8 Parallelism and Performance

f01rgf 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 routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The approximate number of floating-point operations is given by

8 \times m^{2} (n - m)

10 Example

This example reduces the

3 \times 4

matrix

(\begin{array}{r} 2.4 & 0.8 + 0.8 i & - 1.4 + 0.6 i & 3.0 - 1.0 i \\ 0 & 1.6 & 0.8 + 0.3 i & 0.4 + 0.5 i \\ 0 & 0 & 1.0 & 2.0 - 1.0 i \end{array})

to upper triangular form.

f01rg: FL CL CPP AD

NAG FL Interfacef01rgf (complex_​trapez_​rq)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
f01rgf (complex_trapez_rq)