NAG CL Interface
f08atc (zungqr)

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1 Purpose

f08atc generates all or part of the complex unitary matrix Q from a QR factorization computed by f08asc or f08btc.

2 Specification

#include <nag.h>
void  f08atc (Nag_OrderType order, Integer m, Integer n, Integer k, Complex a[], Integer pda, const Complex tau[], NagError *fail)
The function may be called by the names: f08atc, nag_lapackeig_zungqr or nag_zungqr.

3 Description

f08atc is intended to be used after a call to f08asc or f08btc, which perform a QR factorization of a complex matrix A. The unitary matrix Q is represented as a product of elementary reflectors.
This function may be used to generate Q explicitly as a square matrix, or to form only its leading columns.
Usually Q is determined from the QR factorization of an m×p matrix A with mp. The whole of Q may be computed by :
nag_lapackeig_zungqr(order,m,m,p,a,pda,tau,&fail)
(note that the array a must have at least m columns) or its leading p columns by :
nag_lapackeig_zungqr(order,m,p,p,a,pda,tau,&fail)
The columns of Q returned by the last call form an orthonormal basis for the space spanned by the columns of A; thus f08asc followed by f08atc can be used to orthogonalize the columns of A.
The information returned by the QR factorization functions also yields the QR factorization of the leading k columns of A, where k<p. The unitary matrix arising from this factorization can be computed by :
nag_lapackeig_zungqr(order,m,m,k,a,pda,tau,&fail)
or its leading k columns by :
nag_lapackeig_zungqr(order,m,k,k,a,pda,tau,&fail)

4 References

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

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 or Nag_ColMajor.
2: m Integer Input
On entry: m, the order of the unitary matrix Q.
Constraint: m0.
3: n Integer Input
On entry: n, the number of columns of the matrix Q.
Constraint: mn0.
4: k Integer Input
On entry: k, the number of elementary reflectors whose product defines the matrix Q.
Constraint: nk0.
5: a[dim] Complex Input/Output
Note: the dimension, dim, of the array a must be at least
  • max(1,pda×n) when order=Nag_ColMajor;
  • max(1,m×pda) when order=Nag_RowMajor.
On entry: details of the vectors which define the elementary reflectors, as returned by f08asc or f08btc.
On exit: the m×n matrix Q.
6: pda Integer Input
On entry: the stride separating row or column elements (depending on the value of order) in the array a.
Constraints:
  • if order=Nag_ColMajor, pdamax(1,m);
  • if order=Nag_RowMajor, pdamax(1,n).
7: tau[dim] const Complex Input
Note: the dimension, dim, of the array tau must be at least max(1,k).
On entry: further details of the elementary reflectors, as returned by f08asc or f08btc.
8: 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, m=value.
Constraint: m0.
On entry, pda=value.
Constraint: pda>0.
NE_INT_2
On entry, m=value and n=value.
Constraint: mn0.
On entry, n=value and k=value.
Constraint: nk0.
On entry, pda=value and m=value.
Constraint: pdamax(1,m).
On entry, pda=value and n=value.
Constraint: pdamax(1,n).
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 7.5 in the Introduction to the NAG Library CL Interface for further information.
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 computed matrix Q differs from an exactly unitary matrix by a matrix E such that
E2 = O(ε) ,  
where ε is the machine precision.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f08atc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08atc 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

The total number of real floating-point operations is approximately 16mnk-8 (m+n) k2 + 163 k3 ; when n=k, the number is approximately 83 n2 (3m-n) .
The real analogue of this function is f08afc.

10 Example

This example forms the leading 4 columns of the unitary matrix Q from the QR factorization of the matrix A, where
A = ( 0.96-0.81i -0.03+0.96i -0.91+2.06i -0.05+0.41i -0.98+1.98i -1.20+0.19i -0.66+0.42i -0.81+0.56i 0.62-0.46i 1.01+0.02i 0.63-0.17i -1.11+0.60i -0.37+0.38i 0.19-0.54i -0.98-0.36i 0.22-0.20i 0.83+0.51i 0.20+0.01i -0.17-0.46i 1.47+1.59i 1.08-0.28i 0.20-0.12i -0.07+1.23i 0.26+0.26i ) .  
The columns of Q form an orthonormal basis for the space spanned by the columns of A.

10.1 Program Text

Program Text (f08atce.c)

10.2 Program Data

Program Data (f08atce.d)

10.3 Program Results

Program Results (f08atce.r)