cgeqr.f(3) LAPACK cgeqr.f(3)

cgeqr.f


subroutine cgeqr (M, N, A, LDA, T, TSIZE, WORK, LWORK, INFO)
CGEQR

CGEQR

Purpose:

CGEQR computes a QR factorization of a complex M-by-N matrix A:
   A = Q * ( R ),
           ( 0 )
where:
   Q is a M-by-M orthogonal matrix;
   R is an upper-triangular N-by-N matrix;
   0 is a (M-N)-by-N zero matrix, if M > N.

Parameters

M
M is INTEGER
The number of rows of the matrix A.  M >= 0.

N

N is INTEGER
The number of columns of the matrix A.  N >= 0.

A

A is COMPLEX array, dimension (LDA,N)
On entry, the M-by-N matrix A.
On exit, the elements on and above the diagonal of the array
contain the min(M,N)-by-N upper trapezoidal matrix R
(R is upper triangular if M >= N);
the elements below the diagonal are used to store part of the 
data structure to represent Q.

LDA

LDA is INTEGER
The leading dimension of the array A.  LDA >= max(1,M).

T

T is COMPLEX array, dimension (MAX(5,TSIZE))
On exit, if INFO = 0, T(1) returns optimal (or either minimal 
or optimal, if query is assumed) TSIZE. See TSIZE for details.
Remaining T contains part of the data structure used to represent Q.
If one wants to apply or construct Q, then one needs to keep T 
(in addition to A) and pass it to further subroutines.

TSIZE

TSIZE is INTEGER
If TSIZE >= 5, the dimension of the array T.
If TSIZE = -1 or -2, then a workspace query is assumed. The routine
only calculates the sizes of the T and WORK arrays, returns these
values as the first entries of the T and WORK arrays, and no error
message related to T or WORK is issued by XERBLA.
If TSIZE = -1, the routine calculates optimal size of T for the 
optimum performance and returns this value in T(1).
If TSIZE = -2, the routine calculates minimal size of T and 
returns this value in T(1).

WORK

(workspace) COMPLEX array, dimension (MAX(1,LWORK))
On exit, if INFO = 0, WORK(1) contains optimal (or either minimal
or optimal, if query was assumed) LWORK.
See LWORK for details.

LWORK

LWORK is INTEGER
The dimension of the array WORK.
If LWORK = -1 or -2, then a workspace query is assumed. The routine
only calculates the sizes of the T and WORK arrays, returns these
values as the first entries of the T and WORK arrays, and no error
message related to T or WORK is issued by XERBLA.
If LWORK = -1, the routine calculates optimal size of WORK for the
optimal performance and returns this value in WORK(1).
If LWORK = -2, the routine calculates minimal size of WORK and 
returns this value in WORK(1).

INFO

INFO is INTEGER
= 0:  successful exit
< 0:  if INFO = -i, the i-th argument had an illegal value

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Further Details

The goal of the interface is to give maximum freedom to the developers for
creating any QR factorization algorithm they wish. The triangular 
(trapezoidal) R has to be stored in the upper part of A. The lower part of A
and the array T can be used to store any relevant information for applying or
constructing the Q factor. The WORK array can safely be discarded after exit.
Caution: One should not expect the sizes of T and WORK to be the same from one 
LAPACK implementation to the other, or even from one execution to the other.
A workspace query (for T and WORK) is needed at each execution. However, 
for a given execution, the size of T and WORK are fixed and will not change 
from one query to the next.

Further Details particular to this LAPACK implementation:

These details are particular for this LAPACK implementation. Users should not 
take them for granted. These details may change in the future, and are not likely
true for another LAPACK implementation. These details are relevant if one wants
to try to understand the code. They are not part of the interface.
In this version,
         T(2): row block size (MB)
         T(3): column block size (NB)
         T(6:TSIZE): data structure needed for Q, computed by
                          CLATSQR or CGEQRT
 Depending on the matrix dimensions M and N, and row and column
 block sizes MB and NB returned by ILAENV, CGEQR will use either
 CLATSQR (if the matrix is tall-and-skinny) or CGEQRT to compute
 the QR factorization.

Definition at line 172 of file cgeqr.f.

Generated automatically by Doxygen for LAPACK from the source code.
Tue Jun 29 2021 Version 3.10.0