STZRZF(3)      LAPACK routine of NEC Numeric Library Collection      STZRZF(3)



NAME
       STZRZF

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



PURPOSE
            STZRZF reduces the M-by-N ( M<=N ) real upper trapezoidal matrix A
            to upper triangular form by means of orthogonal transformations.

            The upper trapezoidal matrix A is factored as

               A = ( R  0 ) * Z,

            where Z is an N-by-N orthogonal matrix and R is an M-by-M upper
            triangular matrix.




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

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

           A         (input/output)
                     A is REAL array, dimension (LDA,N)
                     On entry, the leading M-by-N upper trapezoidal part of the
                     array A must contain the matrix to be factorized.
                     On exit, the leading M-by-M upper triangular part of A
                     contains the upper triangular matrix R, and elements M+1 to
                     N of the first M rows of A, with the array TAU, represent the
                     orthogonal matrix Z as a product of M elementary reflectors.

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

           TAU       (output)
                     TAU is REAL array, dimension (M)
                     The scalar factors of the elementary reflectors.

           WORK      (output)
                     WORK is REAL array, dimension (MAX(1,LWORK))
                     On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

           LWORK     (input)
                     LWORK is INTEGER
                     The dimension of the array WORK.  LWORK >= max(1,M).
                     For optimum performance LWORK >= M*NB, where NB is
                     the optimal blocksize.

                     If LWORK = -1, then 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 by XERBLA.

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








FURTHER DETAILS
             The N-by-N matrix Z can be computed by

                Z =  Z(1)*Z(2)* ... *Z(M)

             where each N-by-N Z(k) is given by

                Z(k) = I - tau(k)*v(k)*v(k)**T

             with v(k) is the kth row vector of the M-by-N matrix

                V = ( I   A(:,M+1:N) )

             I is the M-by-M identity matrix, A(:,M+1:N)
             is the output stored in A on exit from DTZRZF,
             and tau(k) is the kth element of the array TAU.



LAPACK routine                  31 October 2017                      STZRZF(3)