# slabrd.f man page

slabrd.f —

## Synopsis

### Functions/Subroutines

subroutineslabrd(M, N, NB, A, LDA, D, E, TAUQ, TAUP, X, LDX, Y, LDY)SLABRDreduces the first nb rows and columns of a general matrix to a bidiagonal form.

## Function/Subroutine Documentation

### subroutine slabrd (integerM, integerN, integerNB, real, dimension( lda, * )A, integerLDA, real, dimension( * )D, real, dimension( * )E, real, dimension( * )TAUQ, real, dimension( * )TAUP, real, dimension( ldx, * )X, integerLDX, real, dimension( ldy, * )Y, integerLDY)

**SLABRD** reduces the first nb rows and columns of a general matrix to a bidiagonal form.

**Purpose:**

```
SLABRD reduces the first NB rows and columns of a real general
m by n matrix A to upper or lower bidiagonal form by an orthogonal
transformation Q**T * A * P, and returns the matrices X and Y which
are needed to apply the transformation to the unreduced part of A.
If m >= n, A is reduced to upper bidiagonal form; if m < n, to lower
bidiagonal form.
This is an auxiliary routine called by SGEBRD
```

**Parameters:**

*M*

```
M is INTEGER
The number of rows in the matrix A.
```

*N*

```
N is INTEGER
The number of columns in the matrix A.
```

*NB*

```
NB is INTEGER
The number of leading rows and columns of A to be reduced.
```

*A*

```
A is REAL array, dimension (LDA,N)
On entry, the m by n general matrix to be reduced.
On exit, the first NB rows and columns of the matrix are
overwritten; the rest of the array is unchanged.
If m >= n, elements on and below the diagonal in the first NB
columns, with the array TAUQ, represent the orthogonal
matrix Q as a product of elementary reflectors; and
elements above the diagonal in the first NB rows, with the
array TAUP, represent the orthogonal matrix P as a product
of elementary reflectors.
If m < n, elements below the diagonal in the first NB
columns, with the array TAUQ, represent the orthogonal
matrix Q as a product of elementary reflectors, and
elements on and above the diagonal in the first NB rows,
with the array TAUP, represent the orthogonal matrix P as
a product of elementary reflectors.
See Further Details.
```

*LDA*

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

*D*

```
D is REAL array, dimension (NB)
The diagonal elements of the first NB rows and columns of
the reduced matrix. D(i) = A(i,i).
```

*E*

```
E is REAL array, dimension (NB)
The off-diagonal elements of the first NB rows and columns of
the reduced matrix.
```

*TAUQ*

```
TAUQ is REAL array dimension (NB)
The scalar factors of the elementary reflectors which
represent the orthogonal matrix Q. See Further Details.
```

*TAUP*

```
TAUP is REAL array, dimension (NB)
The scalar factors of the elementary reflectors which
represent the orthogonal matrix P. See Further Details.
```

*X*

```
X is REAL array, dimension (LDX,NB)
The m-by-nb matrix X required to update the unreduced part
of A.
```

*LDX*

```
LDX is INTEGER
The leading dimension of the array X. LDX >= max(1,M).
```

*Y*

```
Y is REAL array, dimension (LDY,NB)
The n-by-nb matrix Y required to update the unreduced part
of A.
```

*LDY*

```
LDY is INTEGER
The leading dimension of the array Y. LDY >= max(1,N).
```

**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:
Q = H(1) H(2) . . . H(nb) and P = G(1) G(2) . . . G(nb)
Each H(i) and G(i) has the form:
H(i) = I - tauq * v * v**T and G(i) = I - taup * u * u**T
where tauq and taup are real scalars, and v and u are real vectors.
If m >= n, v(1:i-1) = 0, v(i) = 1, and v(i:m) is stored on exit in
A(i:m,i); u(1:i) = 0, u(i+1) = 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).
If m < n, v(1:i) = 0, v(i+1) = 1, and v(i+1:m) is stored on exit in
A(i+2:m,i); u(1:i-1) = 0, u(i) = 1, and u(i:n) is stored on exit in
A(i,i+1:n); tauq is stored in TAUQ(i) and taup in TAUP(i).
The elements of the vectors v and u together form the m-by-nb matrix
V and the nb-by-n matrix U**T which are needed, with X and Y, to apply
the transformation to the unreduced part of the matrix, using a block
update of the form: A := A - V*Y**T - X*U**T.
The contents of A on exit are illustrated by the following examples
with nb = 2:
m = 6 and n = 5 (m > n): m = 5 and n = 6 (m < n):
( 1 1 u1 u1 u1 ) ( 1 u1 u1 u1 u1 u1 )
( v1 1 1 u2 u2 ) ( 1 1 u2 u2 u2 u2 )
( v1 v2 a a a ) ( v1 1 a a a a )
( v1 v2 a a a ) ( v1 v2 a a a a )
( v1 v2 a a a ) ( v1 v2 a a a a )
( v1 v2 a a a )
where a denotes an element of the original matrix which is unchanged,
vi denotes an element of the vector defining H(i), and ui an element
of the vector defining G(i).
```

Definition at line 210 of file slabrd.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

slabrd(3) is an alias of slabrd.f(3).