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dgeqr2

 NAME
      DGEQR2 - compute a QR factorization of a real m by n matrix
      A

 SYNOPSIS
      SUBROUTINE DGEQR2( M, N, A, LDA, TAU, WORK, INFO )

          INTEGER        INFO, LDA, M, N

          DOUBLE         PRECISION A( LDA, * ), TAU( * ), WORK( *
                         )

 PURPOSE
      DGEQR2 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) DOUBLE PRECISION array, dimension (LDA,N)
              On entry, the m by n matrix A.  On exit, the ele-
              ments 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 orthogo-
              nal 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) DOUBLE PRECISION array, dimension (min(M,N))
              The scalar factors of the elementary reflectors (see
              Further Details).

      WORK    (workspace) DOUBLE PRECISION 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).