LAPACK  3.4.2
LAPACK: Linear Algebra PACKage
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zlaqr1.f File Reference

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Functions/Subroutines

subroutine zlaqr1 (N, H, LDH, S1, S2, V)
 ZLAQR1 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 zlaqr1 ( integer  N,
complex*16, dimension( ldh, * )  H,
integer  LDH,
complex*16  S1,
complex*16  S2,
complex*16, dimension( * )  V 
)

ZLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

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Purpose:
      Given a 2-by-2 or 3-by-3 matrix H, ZLAQR1 sets v to a
      scalar multiple of the first column of the product

      (*)  K = (H - s1*I)*(H - s2*I)

      scaling to avoid overflows and most underflows.

      This is useful for starting double implicit shift bulges
      in the QR algorithm.
Parameters:
[in]N
          N is integer
              Order of the matrix H. N must be either 2 or 3.
[in]H
          H is COMPLEX*16 array of dimension (LDH,N)
              The 2-by-2 or 3-by-3 matrix H in (*).
[in]LDH
          LDH is integer
              The leading dimension of H as declared in
              the calling procedure.  LDH.GE.N
[in]S1
          S1 is COMPLEX*16
[in]S2
          S2 is COMPLEX*16

          S1 and S2 are the shifts defining K in (*) above.
[out]V
          V is COMPLEX*16 array of dimension N
              A scalar multiple of the first column of the
              matrix K in (*).
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
September 2012
Contributors:
Karen Braman and Ralph Byers, Department of Mathematics, University of Kansas, USA

Definition at line 108 of file zlaqr1.f.

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