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# sgebrd

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NAME
SGEBRD - reduce a general real M-by-N matrix A to upper or
lower bidiagonal form B by an orthogonal transformation

SYNOPSIS
SUBROUTINE SGEBRD( M, N, A, LDA, D, E, TAUQ, TAUP, WORK,
LWORK, INFO )

INTEGER        INFO, LDA, LWORK, M, N

REAL           A( LDA, * ), D( * ), E( * ), TAUP( * ),
TAUQ( * ), WORK( LWORK )

PURPOSE
SGEBRD reduces a general real M-by-N matrix A to upper or
lower bidiagonal form B by an orthogonal transformation:
Q**T * A * P = B.

If m >= n, B is upper bidiagonal; if m < n, B is lower bidi-
agonal.

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

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

A       (input/output) REAL array, dimension (LDA,N)
On entry, the M-by-N general matrix to be reduced.
On exit, if m >= n, the diagonal and the first
superdiagonal are overwritten with the upper bidiag-
onal matrix B; the elements below the diagonal, with
the array TAUQ, represent the orthogonal matrix Q as
a product of elementary reflectors, and the elements
above the first superdiagonal, with the array TAUP,
represent the orthogonal matrix P as a product of
elementary reflectors; if m < n, the diagonal and
the first subdiagonal are overwritten with the lower
bidiagonal matrix B; the elements below the first
subdiagonal, with the array TAUQ, represent the
orthogonal matrix Q as a product of elementary
reflectors, and the elements above the diagonal,
with the array TAUP, represent the orthogonal matrix
P as a product of elementary reflectors.  See
Further Details.  LDA     (input) INTEGER The lead-
ing dimension of the array A.  LDA >= max(1,M).

D       (output) REAL array, dimension (min(M,N))
The diagonal elements of the bidiagonal matrix B:
D(i) = A(i,i).

E       (output) REAL array, dimension (min(M,N)-1)
The off-diagonal elements of the bidiagonal matrix
B: if m >= n, E(i) = A(i,i+1) for i = 1,2,...,n-1;
if m < n, E(i) = A(i+1,i) for i = 1,2,...,m-1.

TAUQ    (output) REAL array dimension (min(M,N))
The scalar factors of the elementary reflectors
which represent the orthogonal matrix Q. See Further
Details.  TAUP    (output) REAL array, dimension
(min(M,N)) The scalar factors of the elementary
reflectors which represent the orthogonal matrix P.
See Further Details.  WORK    (workspace) REAL
array, dimension (LWORK) On exit, if INFO = 0,
WORK(1) returns the optimal LWORK.

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

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

FURTHER DETAILS
The matrices Q and P are represented as products of elemen-
tary reflectors:

If m >= n,

Q = H(1) H(2) . . . H(n)  and  P = G(1) G(2) . . . G(n-1)

Each H(i) and G(i) has the form:

H(i) = I - tauq * v * v'  and G(i) = I - taup * u * u'

where tauq and taup are real scalars, and v and u are real
vectors; v(1:i-1) = 0, v(i) = 1, and v(i+1:m) is stored on
exit in A(i+1:m,i); u(1:i) = 0, u(i+1) = 1, and u(i+2:n) is
stored on exit in A(i,i+2:n); tauq is stored in TAUQ(i) and
taup in TAUP(i).

If m < n,

Q = H(1) H(2) . . . H(m-1)  and  P = G(1) G(2) . . . G(m)

Each H(i) and G(i) has the form:

H(i) = I - tauq * v * v'  and G(i) = I - taup * u * u'

where tauq and taup are real scalars, and v and u are real

vectors; v(1:i) = 0, v(i+1) = 1, and v(i+2:m) is stored on
exit in A(i+2:m,i); u(1:i-1) = 0, u(i) = 1, and u(i+1:n) is
stored on exit in A(i,i+1:n); tauq is stored in TAUQ(i) and
taup in TAUP(i).

The contents of A on exit are illustrated by the following
examples:

m = 6 and n = 5 (m > n):          m = 5 and n = 6 (m < n):

(  d   e   u1  u1  u1 )           (  d   u1  u1  u1  u1
u1 )
(  v1  d   e   u2  u2 )           (  e   d   u2  u2  u2
u2 )
(  v1  v2  d   e   u3 )           (  v1  e   d   u3  u3
u3 )
(  v1  v2  v3  d   e  )           (  v1  v2  e   d   u4
u4 )
(  v1  v2  v3  v4  d  )           (  v1  v2  v3  e   d
u5 )
(  v1  v2  v3  v4  v5 )

where d and e denote diagonal and off-diagonal elements of
B, vi denotes an element of the vector defining H(i), and ui
an element of the vector defining G(i).
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