Annotation of rpl/lapack/lapack/zlaqr1.f, revision 1.8
1.8 ! bertrand 1: *> \brief \b ZLAQR1
! 2: *
! 3: * =========== DOCUMENTATION ===========
! 4: *
! 5: * Online html documentation available at
! 6: * http://www.netlib.org/lapack/explore-html/
! 7: *
! 8: *> \htmlonly
! 9: *> Download ZLAQR1 + dependencies
! 10: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/zlaqr1.f">
! 11: *> [TGZ]</a>
! 12: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/zlaqr1.f">
! 13: *> [ZIP]</a>
! 14: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/zlaqr1.f">
! 15: *> [TXT]</a>
! 16: *> \endhtmlonly
! 17: *
! 18: * Definition:
! 19: * ===========
! 20: *
! 21: * SUBROUTINE ZLAQR1( N, H, LDH, S1, S2, V )
! 22: *
! 23: * .. Scalar Arguments ..
! 24: * COMPLEX*16 S1, S2
! 25: * INTEGER LDH, N
! 26: * ..
! 27: * .. Array Arguments ..
! 28: * COMPLEX*16 H( LDH, * ), V( * )
! 29: * ..
! 30: *
! 31: *
! 32: *> \par Purpose:
! 33: * =============
! 34: *>
! 35: *> \verbatim
! 36: *>
! 37: *> Given a 2-by-2 or 3-by-3 matrix H, ZLAQR1 sets v to a
! 38: *> scalar multiple of the first column of the product
! 39: *>
! 40: *> (*) K = (H - s1*I)*(H - s2*I)
! 41: *>
! 42: *> scaling to avoid overflows and most underflows.
! 43: *>
! 44: *> This is useful for starting double implicit shift bulges
! 45: *> in the QR algorithm.
! 46: *> \endverbatim
! 47: *
! 48: * Arguments:
! 49: * ==========
! 50: *
! 51: *> \param[in] N
! 52: *> \verbatim
! 53: *> N is integer
! 54: *> Order of the matrix H. N must be either 2 or 3.
! 55: *> \endverbatim
! 56: *>
! 57: *> \param[in] H
! 58: *> \verbatim
! 59: *> H is COMPLEX*16 array of dimension (LDH,N)
! 60: *> The 2-by-2 or 3-by-3 matrix H in (*).
! 61: *> \endverbatim
! 62: *>
! 63: *> \param[in] LDH
! 64: *> \verbatim
! 65: *> LDH is integer
! 66: *> The leading dimension of H as declared in
! 67: *> the calling procedure. LDH.GE.N
! 68: *> \endverbatim
! 69: *>
! 70: *> \param[in] S1
! 71: *> \verbatim
! 72: *> S1 is COMPLEX*16
! 73: *> \endverbatim
! 74: *>
! 75: *> \param[in] S2
! 76: *> \verbatim
! 77: *> S2 is COMPLEX*16
! 78: *>
! 79: *> S1 and S2 are the shifts defining K in (*) above.
! 80: *> \endverbatim
! 81: *>
! 82: *> \param[out] V
! 83: *> \verbatim
! 84: *> V is COMPLEX*16 array of dimension N
! 85: *> A scalar multiple of the first column of the
! 86: *> matrix K in (*).
! 87: *> \endverbatim
! 88: *
! 89: * Authors:
! 90: * ========
! 91: *
! 92: *> \author Univ. of Tennessee
! 93: *> \author Univ. of California Berkeley
! 94: *> \author Univ. of Colorado Denver
! 95: *> \author NAG Ltd.
! 96: *
! 97: *> \date November 2011
! 98: *
! 99: *> \ingroup complex16OTHERauxiliary
! 100: *
! 101: *> \par Contributors:
! 102: * ==================
! 103: *>
! 104: *> Karen Braman and Ralph Byers, Department of Mathematics,
! 105: *> University of Kansas, USA
! 106: *>
! 107: * =====================================================================
1.1 bertrand 108: SUBROUTINE ZLAQR1( N, H, LDH, S1, S2, V )
109: *
1.8 ! bertrand 110: * -- LAPACK auxiliary routine (version 3.4.0) --
! 111: * -- LAPACK is a software package provided by Univ. of Tennessee, --
! 112: * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
! 113: * November 2011
1.1 bertrand 114: *
115: * .. Scalar Arguments ..
116: COMPLEX*16 S1, S2
117: INTEGER LDH, N
118: * ..
119: * .. Array Arguments ..
120: COMPLEX*16 H( LDH, * ), V( * )
121: * ..
122: *
1.8 ! bertrand 123: * ================================================================
1.1 bertrand 124: *
125: * .. Parameters ..
126: COMPLEX*16 ZERO
127: PARAMETER ( ZERO = ( 0.0d0, 0.0d0 ) )
128: DOUBLE PRECISION RZERO
129: PARAMETER ( RZERO = 0.0d0 )
130: * ..
131: * .. Local Scalars ..
132: COMPLEX*16 CDUM, H21S, H31S
133: DOUBLE PRECISION S
134: * ..
135: * .. Intrinsic Functions ..
136: INTRINSIC ABS, DBLE, DIMAG
137: * ..
138: * .. Statement Functions ..
139: DOUBLE PRECISION CABS1
140: * ..
141: * .. Statement Function definitions ..
142: CABS1( CDUM ) = ABS( DBLE( CDUM ) ) + ABS( DIMAG( CDUM ) )
143: * ..
144: * .. Executable Statements ..
145: IF( N.EQ.2 ) THEN
146: S = CABS1( H( 1, 1 )-S2 ) + CABS1( H( 2, 1 ) )
147: IF( S.EQ.RZERO ) THEN
148: V( 1 ) = ZERO
149: V( 2 ) = ZERO
150: ELSE
151: H21S = H( 2, 1 ) / S
152: V( 1 ) = H21S*H( 1, 2 ) + ( H( 1, 1 )-S1 )*
153: $ ( ( H( 1, 1 )-S2 ) / S )
154: V( 2 ) = H21S*( H( 1, 1 )+H( 2, 2 )-S1-S2 )
155: END IF
156: ELSE
157: S = CABS1( H( 1, 1 )-S2 ) + CABS1( H( 2, 1 ) ) +
158: $ CABS1( H( 3, 1 ) )
159: IF( S.EQ.ZERO ) THEN
160: V( 1 ) = ZERO
161: V( 2 ) = ZERO
162: V( 3 ) = ZERO
163: ELSE
164: H21S = H( 2, 1 ) / S
165: H31S = H( 3, 1 ) / S
166: V( 1 ) = ( H( 1, 1 )-S1 )*( ( H( 1, 1 )-S2 ) / S ) +
167: $ H( 1, 2 )*H21S + H( 1, 3 )*H31S
168: V( 2 ) = H21S*( H( 1, 1 )+H( 2, 2 )-S1-S2 ) + H( 2, 3 )*H31S
169: V( 3 ) = H31S*( H( 1, 1 )+H( 3, 3 )-S1-S2 ) + H21S*H( 3, 2 )
170: END IF
171: END IF
172: END
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