NAG Library Routine Document

f08ahf (dgelqf)


    1  Purpose
    7  Accuracy


f08ahf (dgelqf) computes the LQ factorization of a real m by n matrix.


Fortran Interface
Subroutine f08ahf ( m, n, a, lda, tau, work, lwork, info)
Integer, Intent (In):: m, n, lda, lwork
Integer, Intent (Out):: info
Real (Kind=nag_wp), Intent (Inout):: a(lda,*), tau(*)
Real (Kind=nag_wp), Intent (Out):: work(max(1,lwork))
C Header Interface
#include nagmk26.h
void  f08ahf_ (const Integer *m, const Integer *n, double a[], const Integer *lda, double tau[], double work[], const Integer *lwork, Integer *info)
The routine may be called by its LAPACK name dgelqf.


f08ahf (dgelqf) forms the LQ factorization of an arbitrary rectangular real m by n matrix. No pivoting is performed.
If mn, the factorization is given by:
A = L 0 Q  
where L is an m by m lower triangular matrix and Q is an n by n orthogonal matrix. It is sometimes more convenient to write the factorization as
A = L 0 Q1 Q2  
which reduces to
A = LQ1 ,  
where Q1 consists of the first m rows of Q, and Q2 the remaining n-m rows.
If m>n, L is trapezoidal, and the factorization can be written
A = L1 L2 Q  
where L1 is lower triangular and L2 is rectangular.
The LQ factorization of A is essentially the same as the QR factorization of AT, since
A = L 0 QAT= QT LT 0 .  
The matrix Q is not formed explicitly but is represented as a product of minm,n elementary reflectors (see the F08 Chapter Introduction for details). Routines are provided to work with Q in this representation (see Section 9).
Note also that for any k<m, the information returned in the first k rows of the array a represents an LQ factorization of the first k rows of the original matrix A.




1:     m – IntegerInput
On entry: m, the number of rows of the matrix A.
Constraint: m0.
2:     n – IntegerInput
On entry: n, the number of columns of the matrix A.
Constraint: n0.
3:     alda* – Real (Kind=nag_wp) arrayInput/Output
Note: the second dimension of the array a must be at least max1,n.
On entry: the m by n matrix A.
On exit: if mn, the elements above the diagonal are overwritten by details of the orthogonal matrix Q and the lower triangle is overwritten by the corresponding elements of the m by m lower triangular matrix L.
If m>n, the strictly upper triangular part is overwritten by details of the orthogonal matrix Q and the remaining elements are overwritten by the corresponding elements of the m by n lower trapezoidal matrix L.
4:     lda – IntegerInput
On entry: the first dimension of the array a as declared in the (sub)program from which f08ahf (dgelqf) is called.
Constraint: ldamax1,m.
5:     tau* – Real (Kind=nag_wp) arrayOutput
Note: the dimension of the array tau must be at least max1,minm,n.
On exit: further details of the orthogonal matrix Q.
6:     workmax1,lwork – Real (Kind=nag_wp) arrayWorkspace
On exit: if info=0, work1 contains the minimum value of lwork required for optimal performance.
7:     lwork – IntegerInput
On entry: the dimension of the array work as declared in the (sub)program from which f08ahf (dgelqf) 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, lworkm×nb, where nb is the optimal block size.
Constraint: lworkmax1,m or lwork=-1.
8:     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 factorization is the exact factorization of a nearby matrix A+E, where
E2 = Oε A2 ,  
and ε is the machine precision.

Parallelism and Performance

f08ahf (dgelqf) 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 floating-point operations is approximately 23 m2 3n-m  if mn or 23 n2 3m-n  if m>n.
To form the orthogonal matrix Q f08ahf (dgelqf) may be followed by a call to f08ajf (dorglq):
Call dorglq(n,n,min(m,n),a,lda,tau,work,lwork,info)
but note that the first dimension of the array a, specified by the argument lda, must be at least n, which may be larger than was required by f08ahf (dgelqf).
When mn, it is often only the first m rows of Q that are required, and they may be formed by the call:
Call dorglq(m,n,m,a,lda,tau,work,lwork,info)
To apply Q to an arbitrary real rectangular matrix C, f08ahf (dgelqf) may be followed by a call to f08akf (dormlq). For example,
Call dormlq('Left','Transpose',m,p,min(m,n),a,lda,tau,c,ldc, &
forms the matrix product C=QTC, where C is m by p.
The complex analogue of this routine is f08avf (zgelqf).


This example finds the minimum norm solutions of the under-determined systems of linear equations
Ax1= b1   and   Ax2= b2  
where b1 and b2 are the columns of the matrix B,
A = -5.42 3.28 -3.68 0.27 2.06 0.46 -1.65 -3.40 -3.20 -1.03 -4.06 -0.01 -0.37 2.35 1.90 4.31 -1.76 1.13 -3.15 -0.11 1.99 -2.70 0.26 4.50   and   B= -2.87 -5.23 1.63 0.29 -3.52 4.76 0.45 -8.41 .  

Program Text

Program Text (f08ahfe.f90)

Program Data

Program Data (f08ahfe.d)

Program Results

Program Results (f08ahfe.r)

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