sgeqr2(3S)

NAME
     SGEQR2 - compute a QR factorization of a real m by n matrix A

SYNOPSIS
     SUBROUTINE SGEQR2( M, N, A, LDA, TAU, WORK, INFO )

         INTEGER        INFO, LDA, M, N

         REAL           A( LDA, * ), TAU( * ), WORK( * )

IMPLEMENTATION
     These routines are part of the SCSL Scientific Library and can be loaded
     using either the -lscs or the -lscs_mp option.  The -lscs_mp option
     directs the linker to use the multi-processor version of the library.

     When linking to SCSL with -lscs or -lscs_mp, the default integer size is
     4 bytes (32 bits). Another version of SCSL is available in which integers
     are 8 bytes (64 bits).  This version allows the user access to larger
     memory sizes and helps when porting legacy Cray codes.  It can be loaded
     by using the -lscs_i8 option or the -lscs_i8_mp option. A program may use
     only one of the two versions; 4-byte integer and 8-byte integer library
     calls cannot be mixed.

PURPOSE
     SGEQR2 computes a QR factorization of a real m by n matrix A: A = Q * R.


ARGUMENTS
     M       (input) INTEGER
             The number of rows of the matrix A.  M >= 0.

     N       (input) INTEGER
             The number of columns of the matrix A.  N >= 0.

     A       (input/output) REAL 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, with the array TAU, represent the
             orthogonal matrix Q as a product of elementary reflectors (see
             Further Details).  LDA     (input) INTEGER The leading dimension
             of the array A.  LDA >= max(1,M).

     TAU     (output) REAL array, dimension (min(M,N))
             The scalar factors of the elementary reflectors (see Further
             Details).

     WORK    (workspace) REAL array, dimension (N)

     INFO    (output) INTEGER
             = 0: successful exit
             < 0: if INFO = -i, the i-th argument had an illegal value
FURTHER DETAILS
     The matrix Q is represented as a product of elementary reflectors

        Q = H(1) H(2) . . . H(k), where k = min(m,n).

     Each H(i) has the form

        H(i) = I - tau * v * v'

     where tau is a real scalar, and v is a real vector with
     v(1:i-1) = 0 and v(i) = 1; v(i+1:m) is stored on exit in A(i+1:m,i), and
     tau in TAU(i).


SEE ALSO
     INTRO_LAPACK(3S), INTRO_SCSL(3S)

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