NAG Library Function Document

nag_zungrq (f08cwc)


    1  Purpose
    7  Accuracy


nag_zungrq (f08cwc) generates all or part of the complex n by n unitary matrix Q from an RQ factorization computed by nag_zgerqf (f08cvc).


#include <nag.h>
#include <nagf08.h>
void  nag_zungrq (Nag_OrderType order, Integer m, Integer n, Integer k, Complex a[], Integer pda, const Complex tau[], NagError *fail)


nag_zungrq (f08cwc) is intended to be used following a call to nag_zgerqf (f08cvc), which performs an RQ factorization of a complex matrix A and represents the unitary matrix Q as a product of k elementary reflectors of order n.
This function may be used to generate Q explicitly as a square matrix, or to form only its trailing rows.
Usually Q is determined from the RQ factorization of a p by n matrix A with pn. The whole of Q may be computed by:
(note that the matrix A must have at least n rows), or its trailing p rows as:
The rows of Q returned by the last call form an orthonormal basis for the space spanned by the rows of A; thus nag_zgerqf (f08cvc) followed by nag_zungrq (f08cwc) can be used to orthogonalize the rows of A.
The information returned by nag_zgerqf (f08cvc) also yields the RQ factorization of the trailing k rows of A, where k<p. The unitary matrix arising from this factorization can be computed by:
or its leading k columns by:


Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999) LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore


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 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:     m IntegerInput
On entry: m, the number of rows of the matrix Q.
Constraint: m0.
3:     n IntegerInput
On entry: n, the number of columns of the matrix Q.
Constraint: nm.
4:     k IntegerInput
On entry: k, the number of elementary reflectors whose product defines the matrix Q.
Constraint: mk0.
5:     a[dim] ComplexInput/Output
Note: the dimension, dim, of the array a must be at least
  • max1,pda×n when order=Nag_ColMajor;
  • max1,m×pda when order=Nag_RowMajor.
On entry: details of the vectors which define the elementary reflectors, as returned by nag_zgerqf (f08cvc).
On exit: the m by n matrix Q.
If order=Nag_ColMajor, the i,jth element of the matrix is stored in a[j-1×pda+i-1].
If order=Nag_RowMajor, the i,jth element of the matrix is stored in a[i-1×pda+j-1].
6:     pda IntegerInput
On entry: the stride separating row or column elements (depending on the value of order) in the array a.
  • if order=Nag_ColMajor, pdamax1,m;
  • if order=Nag_RowMajor, pdamax1,n.
7:     tau[dim] const ComplexInput
Note: the dimension, dim, of the array tau must be at least max1,k.
On entry: tau[i-1] must contain the scalar factor of the elementary reflector Hi, as returned by nag_zgerqf (f08cvc).
8:     fail NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

Error Indicators and Warnings

Dynamic memory allocation failed.
See Section in How to Use the NAG Library and its Documentation for further information.
On entry, argument value had an illegal value.
On entry, m=value.
Constraint: m0.
On entry, pda=value.
Constraint: pda>0.
On entry, m=value and k=value.
Constraint: mk0.
On entry, n=value and m=value.
Constraint: nm.
On entry, pda=value and m=value.
Constraint: pdamax1,m.
On entry, pda=value and n=value.
Constraint: pdamax1,n.
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.
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.


The computed matrix Q differs from an exactly unitary matrix by a matrix E such that
E2 = Oε  
and ε is the machine precision.

Parallelism and Performance

nag_zungrq (f08cwc) 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.

Further Comments

The total number of floating-point operations is approximately 16mnk-8m+nk2+163k3; when m=k this becomes 83m23n-m.
The real analogue of this function is nag_dorgrq (f08cjc).


This example generates the first four rows of the matrix Q of the RQ factorization of A as returned by nag_zgerqf (f08cvc), where
A = 0.96-0.81i -0.98+1.98i 0.62-0.46i -0.37+0.38i 0.83+0.51i 1.08-0.28i -0.03+0.96i -1.20+0.19i 1.01+0.02i 0.19-0.54i 0.20+0.01i 0.20-0.12i -0.91+2.06i -0.66+0.42i 0.63-0.17i -0.98-0.36i -0.17-0.46i -0.07+1.23i -0.05+0.41i -0.81+0.56i -1.11+0.60i 0.22-0.20i 1.47+1.59i 0.26+0.26i .  

Program Text

Program Text (f08cwce.c)

Program Data

Program Data (f08cwce.d)

Program Results

Program Results (f08cwce.r)

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