nag_zunmqr (f08auc) (PDF version)
f08 Chapter Contents
f08 Chapter Introduction
NAG Library Manual

NAG Library Function Document

nag_zunmqr (f08auc)

+ Contents

    1  Purpose
    7  Accuracy
    10  Example

1  Purpose

nag_zunmqr (f08auc) multiplies an arbitrary complex matrix C by the complex unitary matrix Q from a QR factorization computed by nag_zgeqrf (f08asc)nag_zgeqpf (f08bsc) or nag_zgeqp3 (f08btc).

2  Specification

#include <nag.h>
#include <nagf08.h>
void  nag_zunmqr (Nag_OrderType order, Nag_SideType side, Nag_TransType trans, Integer m, Integer n, Integer k, const Complex a[], Integer pda, const Complex tau[], Complex c[], Integer pdc, NagError *fail)

3  Description

nag_zunmqr (f08auc) is intended to be used after a call to nag_zgeqrf (f08asc)nag_zgeqpf (f08bsc) or nag_zgeqp3 (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 form one of the matrix products
QC , QHC , CQ ​ or ​ CQH ,
overwriting the result on c (which may be any complex rectangular matrix).
A common application of this function is in solving linear least squares problems, as described in the f08 Chapter Introduction and illustrated in Section 10 in nag_zgeqrf (f08asc).

4  References

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

5  Arguments

1:     orderNag_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 or Nag_ColMajor.
2:     sideNag_SideTypeInput
On entry: indicates how Q or QH is to be applied to C.
side=Nag_LeftSide
Q or QH is applied to C from the left.
side=Nag_RightSide
Q or QH is applied to C from the right.
Constraint: side=Nag_LeftSide or Nag_RightSide.
3:     transNag_TransTypeInput
On entry: indicates whether Q or QH is to be applied to C.
trans=Nag_NoTrans
Q is applied to C.
trans=Nag_ConjTrans
QH is applied to C.
Constraint: trans=Nag_NoTrans or Nag_ConjTrans.
4:     mIntegerInput
On entry: m, the number of rows of the matrix C.
Constraint: m0.
5:     nIntegerInput
On entry: n, the number of columns of the matrix C.
Constraint: n0.
6:     kIntegerInput
On entry: k, the number of elementary reflectors whose product defines the matrix Q.
Constraints:
  • if side=Nag_LeftSide, m k 0 ;
  • if side=Nag_RightSide, n k 0 .
7:     a[dim]const ComplexInput
Note: the dimension, dim, of the array a must be at least
  • max1,pda×k when order=Nag_ColMajor;
  • max1,m×pda when order=Nag_RowMajor and side=Nag_LeftSide;
  • max1,n×pda when order=Nag_RowMajor and side=Nag_RightSide.
On entry: details of the vectors which define the elementary reflectors, as returned by nag_zgeqrf (f08asc)nag_zgeqpf (f08bsc) or nag_zgeqp3 (f08btc).
8:     pdaIntegerInput
On entry: the stride separating row or column elements (depending on the value of order) in the array a.
Constraints:
  • if order=Nag_ColMajor,
    • if side=Nag_LeftSide, pda max1,m ;
    • if side=Nag_RightSide, pda max1,n ;
  • if order=Nag_RowMajor, pdamax1,k.
9:     tau[dim]const ComplexInput
Note: the dimension, dim, of the array tau must be at least max1,k.
On entry: further details of the elementary reflectors, as returned by nag_zgeqrf (f08asc)nag_zgeqpf (f08bsc) or nag_zgeqp3 (f08btc).
10:   c[dim]ComplexInput/Output
Note: the dimension, dim, of the array c must be at least
  • max1,pdc×n when order=Nag_ColMajor;
  • max1,m×pdc when order=Nag_RowMajor.
The i,jth element of the matrix C is stored in
  • c[j-1×pdc+i-1] when order=Nag_ColMajor;
  • c[i-1×pdc+j-1] when order=Nag_RowMajor.
On entry: the m by n matrix C.
On exit: c is overwritten by QC or QHC or CQ or CQH as specified by side and trans.
11:   pdcIntegerInput
On entry: the stride separating row or column elements (depending on the value of order) in the array c.
Constraints:
  • if order=Nag_ColMajor, pdcmax1,m;
  • if order=Nag_RowMajor, pdcmax1,n.
12:   failNagError *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_ENUM_INT_3
On entry, side=value, m=value, n=value and k=value.
Constraint: if side=Nag_LeftSide, m k 0 ;
if side=Nag_RightSide, n k 0 .
On entry, side=value, m=value, n=value and pda=value.
Constraint: if side=Nag_LeftSide, pda max1,m ;
if side=Nag_RightSide, pda max1,n .
NE_INT
On entry, m=value.
Constraint: m0.
On entry, n=value.
Constraint: n0.
On entry, pda=value.
Constraint: pda>0.
On entry, pdc=value.
Constraint: pdc>0.
NE_INT_2
On entry, pda=value and k=value.
Constraint: pdamax1,k.
On entry, pdc=value and m=value.
Constraint: pdcmax1,m.
On entry, pdc=value and n=value.
Constraint: pdcmax1,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.

7  Accuracy

The computed result differs from the exact result by a matrix E such that
E2 = Oε C2 ,
where ε is the machine precision.

8  Parallelism and Performance

nag_zunmqr (f08auc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_zunmqr (f08auc) 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 Users' Note for your implementation for any additional implementation-specific information.

9  Further Comments

The total number of real floating-point operations is approximately 8nk 2m-k  if side=Nag_LeftSide and 8mk 2n-k  if side=Nag_RightSide.
The real analogue of this function is nag_dormqr (f08agc).

10  Example

See Section 10 in nag_zgeqrf (f08asc).

nag_zunmqr (f08auc) (PDF version)
f08 Chapter Contents
f08 Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2014