372 lines
10 KiB
Fortran
372 lines
10 KiB
Fortran
*> \brief \b SLAGV2 computes the Generalized Schur factorization of a real 2-by-2 matrix pencil (A,B) where B is upper triangular.
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*
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* =========== DOCUMENTATION ===========
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*
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* Online html documentation available at
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* http://www.netlib.org/lapack/explore-html/
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*
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*> \htmlonly
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*> Download SLAGV2 + dependencies
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slagv2.f">
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*> [TGZ]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slagv2.f">
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*> [ZIP]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slagv2.f">
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*> [TXT]</a>
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*> \endhtmlonly
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*
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* Definition:
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* ===========
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*
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* SUBROUTINE SLAGV2( A, LDA, B, LDB, ALPHAR, ALPHAI, BETA, CSL, SNL,
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* CSR, SNR )
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*
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* .. Scalar Arguments ..
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* INTEGER LDA, LDB
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* REAL CSL, CSR, SNL, SNR
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* ..
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* .. Array Arguments ..
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* REAL A( LDA, * ), ALPHAI( 2 ), ALPHAR( 2 ),
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* $ B( LDB, * ), BETA( 2 )
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* ..
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*
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*
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*> \par Purpose:
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* =============
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*>
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*> \verbatim
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*>
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*> SLAGV2 computes the Generalized Schur factorization of a real 2-by-2
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*> matrix pencil (A,B) where B is upper triangular. This routine
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*> computes orthogonal (rotation) matrices given by CSL, SNL and CSR,
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*> SNR such that
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*>
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*> 1) if the pencil (A,B) has two real eigenvalues (include 0/0 or 1/0
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*> types), then
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*>
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*> [ a11 a12 ] := [ CSL SNL ] [ a11 a12 ] [ CSR -SNR ]
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*> [ 0 a22 ] [ -SNL CSL ] [ a21 a22 ] [ SNR CSR ]
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*>
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*> [ b11 b12 ] := [ CSL SNL ] [ b11 b12 ] [ CSR -SNR ]
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*> [ 0 b22 ] [ -SNL CSL ] [ 0 b22 ] [ SNR CSR ],
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*>
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*> 2) if the pencil (A,B) has a pair of complex conjugate eigenvalues,
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*> then
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*>
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*> [ a11 a12 ] := [ CSL SNL ] [ a11 a12 ] [ CSR -SNR ]
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*> [ a21 a22 ] [ -SNL CSL ] [ a21 a22 ] [ SNR CSR ]
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*>
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*> [ b11 0 ] := [ CSL SNL ] [ b11 b12 ] [ CSR -SNR ]
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*> [ 0 b22 ] [ -SNL CSL ] [ 0 b22 ] [ SNR CSR ]
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*>
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*> where b11 >= b22 > 0.
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*>
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*> \endverbatim
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*
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* Arguments:
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* ==========
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*
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*> \param[in,out] A
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*> \verbatim
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*> A is REAL array, dimension (LDA, 2)
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*> On entry, the 2 x 2 matrix A.
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*> On exit, A is overwritten by the ``A-part'' of the
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*> generalized Schur form.
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*> \endverbatim
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*>
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*> \param[in] LDA
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*> \verbatim
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*> LDA is INTEGER
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*> THe leading dimension of the array A. LDA >= 2.
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*> \endverbatim
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*>
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*> \param[in,out] B
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*> \verbatim
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*> B is REAL array, dimension (LDB, 2)
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*> On entry, the upper triangular 2 x 2 matrix B.
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*> On exit, B is overwritten by the ``B-part'' of the
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*> generalized Schur form.
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*> \endverbatim
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*>
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*> \param[in] LDB
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*> \verbatim
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*> LDB is INTEGER
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*> THe leading dimension of the array B. LDB >= 2.
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*> \endverbatim
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*>
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*> \param[out] ALPHAR
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*> \verbatim
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*> ALPHAR is REAL array, dimension (2)
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*> \endverbatim
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*>
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*> \param[out] ALPHAI
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*> \verbatim
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*> ALPHAI is REAL array, dimension (2)
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*> \endverbatim
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*>
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*> \param[out] BETA
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*> \verbatim
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*> BETA is REAL array, dimension (2)
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*> (ALPHAR(k)+i*ALPHAI(k))/BETA(k) are the eigenvalues of the
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*> pencil (A,B), k=1,2, i = sqrt(-1). Note that BETA(k) may
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*> be zero.
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*> \endverbatim
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*>
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*> \param[out] CSL
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*> \verbatim
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*> CSL is REAL
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*> The cosine of the left rotation matrix.
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*> \endverbatim
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*>
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*> \param[out] SNL
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*> \verbatim
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*> SNL is REAL
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*> The sine of the left rotation matrix.
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*> \endverbatim
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*>
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*> \param[out] CSR
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*> \verbatim
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*> CSR is REAL
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*> The cosine of the right rotation matrix.
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*> \endverbatim
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*>
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*> \param[out] SNR
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*> \verbatim
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*> SNR is REAL
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*> The sine of the right rotation matrix.
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*> \endverbatim
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*
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* Authors:
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* ========
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*
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*> \author Univ. of Tennessee
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*> \author Univ. of California Berkeley
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*> \author Univ. of Colorado Denver
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*> \author NAG Ltd.
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*
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*> \ingroup realOTHERauxiliary
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*
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*> \par Contributors:
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* ==================
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*>
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*> Mark Fahey, Department of Mathematics, Univ. of Kentucky, USA
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*
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* =====================================================================
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SUBROUTINE SLAGV2( A, LDA, B, LDB, ALPHAR, ALPHAI, BETA, CSL, SNL,
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$ CSR, SNR )
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*
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* -- LAPACK auxiliary routine --
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* -- LAPACK is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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*
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* .. Scalar Arguments ..
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INTEGER LDA, LDB
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REAL CSL, CSR, SNL, SNR
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* ..
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* .. Array Arguments ..
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REAL A( LDA, * ), ALPHAI( 2 ), ALPHAR( 2 ),
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$ B( LDB, * ), BETA( 2 )
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* ..
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*
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* =====================================================================
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*
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* .. Parameters ..
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REAL ZERO, ONE
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PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0 )
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* ..
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* .. Local Scalars ..
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REAL ANORM, ASCALE, BNORM, BSCALE, H1, H2, H3, QQ,
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$ R, RR, SAFMIN, SCALE1, SCALE2, T, ULP, WI, WR1,
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$ WR2
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* ..
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* .. External Subroutines ..
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EXTERNAL SLAG2, SLARTG, SLASV2, SROT
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* ..
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* .. External Functions ..
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REAL SLAMCH, SLAPY2
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EXTERNAL SLAMCH, SLAPY2
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS, MAX
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* ..
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* .. Executable Statements ..
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*
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SAFMIN = SLAMCH( 'S' )
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ULP = SLAMCH( 'P' )
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*
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* Scale A
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*
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ANORM = MAX( ABS( A( 1, 1 ) )+ABS( A( 2, 1 ) ),
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$ ABS( A( 1, 2 ) )+ABS( A( 2, 2 ) ), SAFMIN )
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ASCALE = ONE / ANORM
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A( 1, 1 ) = ASCALE*A( 1, 1 )
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A( 1, 2 ) = ASCALE*A( 1, 2 )
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A( 2, 1 ) = ASCALE*A( 2, 1 )
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A( 2, 2 ) = ASCALE*A( 2, 2 )
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*
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* Scale B
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*
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BNORM = MAX( ABS( B( 1, 1 ) ), ABS( B( 1, 2 ) )+ABS( B( 2, 2 ) ),
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$ SAFMIN )
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BSCALE = ONE / BNORM
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B( 1, 1 ) = BSCALE*B( 1, 1 )
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B( 1, 2 ) = BSCALE*B( 1, 2 )
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B( 2, 2 ) = BSCALE*B( 2, 2 )
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*
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* Check if A can be deflated
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*
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IF( ABS( A( 2, 1 ) ).LE.ULP ) THEN
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CSL = ONE
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SNL = ZERO
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CSR = ONE
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SNR = ZERO
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A( 2, 1 ) = ZERO
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B( 2, 1 ) = ZERO
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WI = ZERO
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*
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* Check if B is singular
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*
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ELSE IF( ABS( B( 1, 1 ) ).LE.ULP ) THEN
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CALL SLARTG( A( 1, 1 ), A( 2, 1 ), CSL, SNL, R )
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CSR = ONE
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SNR = ZERO
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CALL SROT( 2, A( 1, 1 ), LDA, A( 2, 1 ), LDA, CSL, SNL )
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CALL SROT( 2, B( 1, 1 ), LDB, B( 2, 1 ), LDB, CSL, SNL )
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A( 2, 1 ) = ZERO
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B( 1, 1 ) = ZERO
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B( 2, 1 ) = ZERO
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WI = ZERO
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*
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ELSE IF( ABS( B( 2, 2 ) ).LE.ULP ) THEN
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CALL SLARTG( A( 2, 2 ), A( 2, 1 ), CSR, SNR, T )
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SNR = -SNR
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CALL SROT( 2, A( 1, 1 ), 1, A( 1, 2 ), 1, CSR, SNR )
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CALL SROT( 2, B( 1, 1 ), 1, B( 1, 2 ), 1, CSR, SNR )
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CSL = ONE
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SNL = ZERO
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A( 2, 1 ) = ZERO
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B( 2, 1 ) = ZERO
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B( 2, 2 ) = ZERO
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WI = ZERO
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*
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ELSE
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*
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* B is nonsingular, first compute the eigenvalues of (A,B)
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*
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CALL SLAG2( A, LDA, B, LDB, SAFMIN, SCALE1, SCALE2, WR1, WR2,
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$ WI )
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*
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IF( WI.EQ.ZERO ) THEN
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*
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* two real eigenvalues, compute s*A-w*B
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*
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H1 = SCALE1*A( 1, 1 ) - WR1*B( 1, 1 )
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H2 = SCALE1*A( 1, 2 ) - WR1*B( 1, 2 )
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H3 = SCALE1*A( 2, 2 ) - WR1*B( 2, 2 )
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*
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RR = SLAPY2( H1, H2 )
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QQ = SLAPY2( SCALE1*A( 2, 1 ), H3 )
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*
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IF( RR.GT.QQ ) THEN
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*
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* find right rotation matrix to zero 1,1 element of
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* (sA - wB)
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*
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CALL SLARTG( H2, H1, CSR, SNR, T )
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*
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ELSE
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*
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* find right rotation matrix to zero 2,1 element of
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* (sA - wB)
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*
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CALL SLARTG( H3, SCALE1*A( 2, 1 ), CSR, SNR, T )
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*
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END IF
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*
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SNR = -SNR
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CALL SROT( 2, A( 1, 1 ), 1, A( 1, 2 ), 1, CSR, SNR )
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CALL SROT( 2, B( 1, 1 ), 1, B( 1, 2 ), 1, CSR, SNR )
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*
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* compute inf norms of A and B
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*
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H1 = MAX( ABS( A( 1, 1 ) )+ABS( A( 1, 2 ) ),
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$ ABS( A( 2, 1 ) )+ABS( A( 2, 2 ) ) )
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H2 = MAX( ABS( B( 1, 1 ) )+ABS( B( 1, 2 ) ),
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$ ABS( B( 2, 1 ) )+ABS( B( 2, 2 ) ) )
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*
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IF( ( SCALE1*H1 ).GE.ABS( WR1 )*H2 ) THEN
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*
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* find left rotation matrix Q to zero out B(2,1)
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*
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CALL SLARTG( B( 1, 1 ), B( 2, 1 ), CSL, SNL, R )
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*
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ELSE
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*
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* find left rotation matrix Q to zero out A(2,1)
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*
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CALL SLARTG( A( 1, 1 ), A( 2, 1 ), CSL, SNL, R )
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*
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END IF
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*
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CALL SROT( 2, A( 1, 1 ), LDA, A( 2, 1 ), LDA, CSL, SNL )
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CALL SROT( 2, B( 1, 1 ), LDB, B( 2, 1 ), LDB, CSL, SNL )
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*
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A( 2, 1 ) = ZERO
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B( 2, 1 ) = ZERO
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*
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ELSE
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*
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* a pair of complex conjugate eigenvalues
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* first compute the SVD of the matrix B
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*
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CALL SLASV2( B( 1, 1 ), B( 1, 2 ), B( 2, 2 ), R, T, SNR,
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$ CSR, SNL, CSL )
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*
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* Form (A,B) := Q(A,B)Z**T where Q is left rotation matrix and
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* Z is right rotation matrix computed from SLASV2
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*
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CALL SROT( 2, A( 1, 1 ), LDA, A( 2, 1 ), LDA, CSL, SNL )
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CALL SROT( 2, B( 1, 1 ), LDB, B( 2, 1 ), LDB, CSL, SNL )
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CALL SROT( 2, A( 1, 1 ), 1, A( 1, 2 ), 1, CSR, SNR )
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CALL SROT( 2, B( 1, 1 ), 1, B( 1, 2 ), 1, CSR, SNR )
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*
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B( 2, 1 ) = ZERO
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B( 1, 2 ) = ZERO
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*
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END IF
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*
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END IF
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*
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* Unscaling
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*
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A( 1, 1 ) = ANORM*A( 1, 1 )
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A( 2, 1 ) = ANORM*A( 2, 1 )
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A( 1, 2 ) = ANORM*A( 1, 2 )
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A( 2, 2 ) = ANORM*A( 2, 2 )
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B( 1, 1 ) = BNORM*B( 1, 1 )
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B( 2, 1 ) = BNORM*B( 2, 1 )
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B( 1, 2 ) = BNORM*B( 1, 2 )
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B( 2, 2 ) = BNORM*B( 2, 2 )
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*
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IF( WI.EQ.ZERO ) THEN
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ALPHAR( 1 ) = A( 1, 1 )
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ALPHAR( 2 ) = A( 2, 2 )
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ALPHAI( 1 ) = ZERO
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ALPHAI( 2 ) = ZERO
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BETA( 1 ) = B( 1, 1 )
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BETA( 2 ) = B( 2, 2 )
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ELSE
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ALPHAR( 1 ) = ANORM*WR1 / SCALE1 / BNORM
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ALPHAI( 1 ) = ANORM*WI / SCALE1 / BNORM
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ALPHAR( 2 ) = ALPHAR( 1 )
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ALPHAI( 2 ) = -ALPHAI( 1 )
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BETA( 1 ) = ONE
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BETA( 2 ) = ONE
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END IF
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*
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RETURN
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*
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* End of SLAGV2
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*
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END
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