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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