NAG Library Routine Document

f08ktf (zungbr)


    1  Purpose
    7  Accuracy


f08ktf (zungbr) generates one of the complex unitary matrices Q or PH which were determined by f08ksf (zgebrd) when reducing a complex matrix to bidiagonal form.


Fortran Interface
Subroutine f08ktf ( vect, m, n, k, a, lda, tau, work, lwork, info)
Integer, Intent (In):: m, n, k, lda, lwork
Integer, Intent (Out):: info
Complex (Kind=nag_wp), Intent (In):: tau(*)
Complex (Kind=nag_wp), Intent (Inout):: a(lda,*)
Complex (Kind=nag_wp), Intent (Out):: work(max(1,lwork))
Character (1), Intent (In):: vect
C Header Interface
#include nagmk26.h
void  f08ktf_ (const char *vect, const Integer *m, const Integer *n, const Integer *k, Complex a[], const Integer *lda, const Complex tau[], Complex work[], const Integer *lwork, Integer *info, const Charlen length_vect)
The routine may be called by its LAPACK name zungbr.


f08ktf (zungbr) is intended to be used after a call to f08ksf (zgebrd), which reduces a complex rectangular matrix A to real bidiagonal form B by a unitary transformation: A=QBPH. f08ksf (zgebrd) represents the matrices Q and PH as products of elementary reflectors.
This routine may be used to generate Q or PH explicitly as square matrices, or in some cases just the leading columns of Q or the leading rows of PH.
The various possibilities are specified by the arguments vect, m, n and k. The appropriate values to cover the most likely cases are as follows (assuming that A was an m by n matrix):
1. To form the full m by m matrix Q:
Call zungbr('Q',m,m,n,...)
(note that the array a must have at least m columns).
2. If m>n, to form the n leading columns of Q:
Call zungbr('Q',m,n,n,...)
3. To form the full n by n matrix PH:
Call zungbr('P',n,n,m,...)
(note that the array a must have at least n rows).
4. If m<n, to form the m leading rows of PH:
Call zungbr('P',m,n,m,...)


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


1:     vect – Character(1)Input
On entry: indicates whether the unitary matrix Q or PH is generated.
Q is generated.
PH is generated.
Constraint: vect='Q' or 'P'.
2:     m – IntegerInput
On entry: m, the number of rows of the unitary matrix Q or PH to be returned.
Constraint: m0.
3:     n – IntegerInput
On entry: n, the number of columns of the unitary matrix Q or PH to be returned.
  • n0;
  • if vect='Q' and m>k, mnk;
  • if vect='Q' and mk, m=n;
  • if vect='P' and n>k, nmk;
  • if vect='P' and nk, n=m.
4:     k – IntegerInput
On entry: if vect='Q', the number of columns in the original matrix A.
If vect='P', the number of rows in the original matrix A.
Constraint: k0.
5:     alda* – Complex (Kind=nag_wp) arrayInput/Output
Note: the second dimension of the array a must be at least max1,n.
On entry: details of the vectors which define the elementary reflectors, as returned by f08ksf (zgebrd).
On exit: the unitary matrix Q or PH, or the leading rows or columns thereof, as specified by vect, m and n.
6:     lda – IntegerInput
On entry: the first dimension of the array a as declared in the (sub)program from which f08ktf (zungbr) is called.
Constraint: ldamax1,m.
7:     tau* – Complex (Kind=nag_wp) arrayInput
Note: the dimension of the array tau must be at least max1,minm,k if vect='Q' and at least max1,minn,k if vect='P'.
On entry: further details of the elementary reflectors, as returned by f08ksf (zgebrd) in its argument tauq if vect='Q', or in its argument taup if vect='P'.
8:     workmax1,lwork – Complex (Kind=nag_wp) arrayWorkspace
On exit: if info=0, the real part of work1 contains the minimum value of lwork required for optimal performance.
9:     lwork – IntegerInput
On entry: the dimension of the array work as declared in the (sub)program from which f08ktf (zungbr) is called.
If lwork=-1, a workspace query is assumed; the routine only calculates the optimal size of the work array, returns this value as the first entry of the work array, and no error message related to lwork is issued.
Suggested value: for optimal performance, lworkminm,n×nb, where nb is the optimal block size.
Constraint: lwork max1,minm,n or lwork=-1.
10:   info – IntegerOutput
On exit: info=0 unless the routine detects an error (see Section 6).

Error Indicators and Warnings

If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.


The computed matrix Q differs from an exactly unitary matrix by a matrix E such that
E2 = Oε ,  
where ε is the machine precision. A similar statement holds for the computed matrix PH.

Parallelism and Performance

f08ktf (zungbr) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08ktf (zungbr) 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.

Further Comments

The total number of real floating-point operations for the cases listed in Section 3 are approximately as follows:
1. To form the whole of Q:
  • 163n3m2-3mn+n2 if m>n,
  • 163m3 if mn;
2. To form the n leading columns of Q when m>n:
  • 83n23m-n;
3. To form the whole of PH:
  • 163n3 if mn,
  • 163m33n2-3mn+m2 if m<n;
4. To form the m leading rows of PH when m<n:
  • 83m23n-m.
The real analogue of this routine is f08kff (dorgbr).


For this routine two examples are presented, both of which involve computing the singular value decomposition of a 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  
in the first example and
A = 0.28-0.36i 0.50-0.86i -0.77-0.48i 1.58+0.66i -0.50-1.10i -1.21+0.76i -0.32-0.24i -0.27-1.15i 0.36-0.51i -0.07+1.33i -0.75+0.47i -0.08+1.01i  
in the second. A must first be reduced to tridiagonal form by f08ksf (zgebrd). The program then calls f08ktf (zungbr) twice to form Q and PH, and passes these matrices to f08msf (zbdsqr), which computes the singular value decomposition of A.

Program Text

Program Text (f08ktfe.f90)

Program Data

Program Data (f08ktfe.d)

Program Results

Program Results (f08ktfe.r)

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