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.

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 (*).

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

June 2017

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