LAPACK 3.11.0
LAPACK: Linear Algebra PACKage
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◆ claqr1()

subroutine claqr1 ( integer  N,
complex, dimension( ldh, * )  H,
integer  LDH,
complex  S1,
complex  S2,
complex, dimension( * )  V 
)

CLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

Download CLAQR1 + dependencies [TGZ] [ZIP] [TXT]

Purpose:
      Given a 2-by-2 or 3-by-3 matrix H, CLAQR1 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 array, 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 >= N
[in]S1
          S1 is COMPLEX
[in]S2
          S2 is COMPLEX

          S1 and S2 are the shifts defining K in (*) above.
[out]V
          V is COMPLEX array, 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.
Contributors:
Karen Braman and Ralph Byers, Department of Mathematics, University of Kansas, USA

Definition at line 106 of file claqr1.f.

107*
108* -- LAPACK auxiliary routine --
109* -- LAPACK is a software package provided by Univ. of Tennessee, --
110* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
111*
112* .. Scalar Arguments ..
113 COMPLEX S1, S2
114 INTEGER LDH, N
115* ..
116* .. Array Arguments ..
117 COMPLEX H( LDH, * ), V( * )
118* ..
119*
120* ================================================================
121*
122* .. Parameters ..
123 COMPLEX ZERO
124 parameter( zero = ( 0.0e0, 0.0e0 ) )
125 REAL RZERO
126 parameter( rzero = 0.0e0 )
127* ..
128* .. Local Scalars ..
129 COMPLEX CDUM, H21S, H31S
130 REAL S
131* ..
132* .. Intrinsic Functions ..
133 INTRINSIC abs, aimag, real
134* ..
135* .. Statement Functions ..
136 REAL CABS1
137* ..
138* .. Statement Function definitions ..
139 cabs1( cdum ) = abs( real( cdum ) ) + abs( aimag( cdum ) )
140* ..
141* .. Executable Statements ..
142*
143* Quick return if possible
144*
145 IF( n.NE.2 .AND. n.NE.3 ) THEN
146 RETURN
147 END IF
148*
149 IF( n.EQ.2 ) THEN
150 s = cabs1( h( 1, 1 )-s2 ) + cabs1( h( 2, 1 ) )
151 IF( s.EQ.rzero ) THEN
152 v( 1 ) = zero
153 v( 2 ) = zero
154 ELSE
155 h21s = h( 2, 1 ) / s
156 v( 1 ) = h21s*h( 1, 2 ) + ( h( 1, 1 )-s1 )*
157 $ ( ( h( 1, 1 )-s2 ) / s )
158 v( 2 ) = h21s*( h( 1, 1 )+h( 2, 2 )-s1-s2 )
159 END IF
160 ELSE
161 s = cabs1( h( 1, 1 )-s2 ) + cabs1( h( 2, 1 ) ) +
162 $ cabs1( h( 3, 1 ) )
163 IF( s.EQ.zero ) THEN
164 v( 1 ) = zero
165 v( 2 ) = zero
166 v( 3 ) = zero
167 ELSE
168 h21s = h( 2, 1 ) / s
169 h31s = h( 3, 1 ) / s
170 v( 1 ) = ( h( 1, 1 )-s1 )*( ( h( 1, 1 )-s2 ) / s ) +
171 $ h( 1, 2 )*h21s + h( 1, 3 )*h31s
172 v( 2 ) = h21s*( h( 1, 1 )+h( 2, 2 )-s1-s2 ) + h( 2, 3 )*h31s
173 v( 3 ) = h31s*( h( 1, 1 )+h( 3, 3 )-s1-s2 ) + h21s*h( 3, 2 )
174 END IF
175 END IF
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