dlaqr1.f

Section: LAPACK (3)
Updated: Tue Nov 14 2017
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dlaqr1.f

SYNOPSIS

Functions/Subroutines

subroutine dlaqr1 (N, H, LDH, SR1, SI1, SR2, SI2, V)
DLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

Function/Subroutine Documentation

subroutine dlaqr1 (integer N, double precision, dimension( ldh, * ) H, integer LDH, double precision SR1, double precision SI1, double precision SR2, double precision SI2, double precision, dimension( * ) V)

DLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

Purpose:

```      Given a 2-by-2 or 3-by-3 matrix H, DLAQR1 sets v to a
scalar multiple of the first column of the product

(*)  K = (H - (sr1 + i*si1)*I)*(H - (sr2 + i*si2)*I)

scaling to avoid overflows and most underflows. It
is assumed that either

1) sr1 = sr2 and si1 = -si2
or
2) si1 = si2 = 0.

This is useful for starting double implicit shift bulges
in the QR algorithm.
```

Parameters:

N

```          N is INTEGER
Order of the matrix H. N must be either 2 or 3.
```

H

```          H is DOUBLE PRECISION array, dimension (LDH,N)
The 2-by-2 or 3-by-3 matrix H in (*).
```

LDH

```          LDH is INTEGER
The leading dimension of H as declared in
the calling procedure.  LDH.GE.N
```

SR1

```          SR1 is DOUBLE PRECISION
```

SI1

```          SI1 is DOUBLE PRECISION
```

SR2

```          SR2 is DOUBLE PRECISION
```

SI2

```          SI2 is DOUBLE PRECISION
The shifts in (*).
```

V

```          V is DOUBLE PRECISION array, dimension (N)
A scalar multiple of the first column of the
matrix K in (*).
```

Author:

Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Date:

June 2017

Contributors:

Karen Braman and Ralph Byers, Department of Mathematics, University of Kansas, USA

Definition at line 123 of file dlaqr1.f.

Author

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