slaqr1.f3

!>
!>      Given a 2-by-2 or 3-by-3 matrix H, SLAQR1 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.
!> 

N

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

H

!>          H is REAL 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 >= N
!> 

SR1

!>          SR1 is REAL
!> 

SI1

!>          SI1 is REAL
!> 

SR2

!>          SR2 is REAL
!> 

SI2

!>          SI2 is REAL
!>              The shifts in (*).
!> 

V

!>          V is REAL array, dimension (N)
!>              A scalar multiple of the first column of the
!>              matrix K in (*).
!> 

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

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