CGEBD2 (3)  Linux Man Pages
NAME
cgebd2.f 
SYNOPSIS
Functions/Subroutines
subroutine cgebd2 (M, N, A, LDA, D, E, TAUQ, TAUP, WORK, INFO)
CGEBD2 reduces a general matrix to bidiagonal form using an unblocked algorithm.
Function/Subroutine Documentation
subroutine cgebd2 (integerM, integerN, complex, dimension( lda, * )A, integerLDA, real, dimension( * )D, real, dimension( * )E, complex, dimension( * )TAUQ, complex, dimension( * )TAUP, complex, dimension( * )WORK, integerINFO)
CGEBD2 reduces a general matrix to bidiagonal form using an unblocked algorithm.
Purpose:

CGEBD2 reduces a complex general m by n matrix A to upper or lower real bidiagonal form B by a unitary transformation: Q**H * A * P = B. If m >= n, B is upper bidiagonal; if m < n, B is lower bidiagonal.
Parameters:

M
M is INTEGER The number of rows in the matrix A. M >= 0.
NN is INTEGER The number of columns in the matrix A. N >= 0.
AA is COMPLEX 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 bidiagonal matrix B; the elements below the diagonal, with the array TAUQ, represent the unitary matrix Q as a product of elementary reflectors, and the elements above the first superdiagonal, with the array TAUP, represent the unitary 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 unitary matrix Q as a product of elementary reflectors, and the elements above the diagonal, with the array TAUP, represent the unitary matrix P as a product of elementary reflectors. See Further Details.
LDALDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).
DD is REAL array, dimension (min(M,N)) The diagonal elements of the bidiagonal matrix B: D(i) = A(i,i).
EE is REAL array, dimension (min(M,N)1) The offdiagonal elements of the bidiagonal matrix B: if m >= n, E(i) = A(i,i+1) for i = 1,2,...,n1; if m < n, E(i) = A(i+1,i) for i = 1,2,...,m1.
TAUQTAUQ is COMPLEX array dimension (min(M,N)) The scalar factors of the elementary reflectors which represent the unitary matrix Q. See Further Details.
TAUPTAUP is COMPLEX array, dimension (min(M,N)) The scalar factors of the elementary reflectors which represent the unitary matrix P. See Further Details.
WORKWORK is COMPLEX array, dimension (max(M,N))
INFOINFO is INTEGER = 0: successful exit < 0: if INFO = i, the ith argument had an illegal value.
Author:

Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
 September 2012
Further Details:

The matrices Q and P are represented as products of elementary reflectors: If m >= n, Q = H(1) H(2) . . . H(n) and P = G(1) G(2) . . . G(n1) Each H(i) and G(i) has the form: H(i) = I  tauq * v * v**H and G(i) = I  taup * u * u**H where tauq and taup are complex scalars, and v and u are complex vectors; v(1:i1) = 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(m1) and P = G(1) G(2) . . . G(m) Each H(i) and G(i) has the form: H(i) = I  tauq * v * v**H and G(i) = I  taup * u * u**H where tauq and taup are complex scalars, v and u are complex 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:i1) = 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 offdiagonal elements of B, vi denotes an element of the vector defining H(i), and ui an element of the vector defining G(i).
Definition at line 191 of file cgebd2.f.
Author
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