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*> \brief \b SLASV2 computes the singular value decomposition of a 2-by-2 triangular matrix.
*
* =========== DOCUMENTATION ===========
*
* Online html documentation available at
* http://www.netlib.org/lapack/explore-html/
*
*> \htmlonly
*> Download SLASV2 + dependencies
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slasv2.f">
*> [TGZ]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slasv2.f">
*> [ZIP]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slasv2.f">
*> [TXT]</a>
*> \endhtmlonly
*
* Definition:
* ===========
*
* SUBROUTINE SLASV2( F, G, H, SSMIN, SSMAX, SNR, CSR, SNL, CSL )
*
* .. Scalar Arguments ..
* REAL CSL, CSR, F, G, H, SNL, SNR, SSMAX, SSMIN
* ..
*
*
*> \par Purpose:
* =============
*>
*> \verbatim
*>
*> SLASV2 computes the singular value decomposition of a 2-by-2
*> triangular matrix
*> [ F G ]
*> [ 0 H ].
*> On return, abs(SSMAX) is the larger singular value, abs(SSMIN) is the
*> smaller singular value, and (CSL,SNL) and (CSR,SNR) are the left and
*> right singular vectors for abs(SSMAX), giving the decomposition
*>
*> [ CSL SNL ] [ F G ] [ CSR -SNR ] = [ SSMAX 0 ]
*> [-SNL CSL ] [ 0 H ] [ SNR CSR ] [ 0 SSMIN ].
*> \endverbatim
*
* Arguments:
* ==========
*
*> \param[in] F
*> \verbatim
*> F is REAL
*> The (1,1) element of the 2-by-2 matrix.
*> \endverbatim
*>
*> \param[in] G
*> \verbatim
*> G is REAL
*> The (1,2) element of the 2-by-2 matrix.
*> \endverbatim
*>
*> \param[in] H
*> \verbatim
*> H is REAL
*> The (2,2) element of the 2-by-2 matrix.
*> \endverbatim
*>
*> \param[out] SSMIN
*> \verbatim
*> SSMIN is REAL
*> abs(SSMIN) is the smaller singular value.
*> \endverbatim
*>
*> \param[out] SSMAX
*> \verbatim
*> SSMAX is REAL
*> abs(SSMAX) is the larger singular value.
*> \endverbatim
*>
*> \param[out] SNL
*> \verbatim
*> SNL is REAL
*> \endverbatim
*>
*> \param[out] CSL
*> \verbatim
*> CSL is REAL
*> The vector (CSL, SNL) is a unit left singular vector for the
*> singular value abs(SSMAX).
*> \endverbatim
*>
*> \param[out] SNR
*> \verbatim
*> SNR is REAL
*> \endverbatim
*>
*> \param[out] CSR
*> \verbatim
*> CSR is REAL
*> The vector (CSR, SNR) is a unit right singular vector for the
*> singular value abs(SSMAX).
*> \endverbatim
*
* Authors:
* ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \date December 2016
*
*> \ingroup OTHERauxiliary
*
*> \par Further Details:
* =====================
*>
*> \verbatim
*>
*> Any input parameter may be aliased with any output parameter.
*>
*> Barring over/underflow and assuming a guard digit in subtraction, all
*> output quantities are correct to within a few units in the last
*> place (ulps).
*>
*> In IEEE arithmetic, the code works correctly if one matrix element is
*> infinite.
*>
*> Overflow will not occur unless the largest singular value itself
*> overflows or is within a few ulps of overflow. (On machines with
*> partial overflow, like the Cray, overflow may occur if the largest
*> singular value is within a factor of 2 of overflow.)
*>
*> Underflow is harmless if underflow is gradual. Otherwise, results
*> may correspond to a matrix modified by perturbations of size near
*> the underflow threshold.
*> \endverbatim
*>
* =====================================================================
SUBROUTINE SLASV2( F, G, H, SSMIN, SSMAX, SNR, CSR, SNL, CSL )
*
* -- LAPACK auxiliary routine (version 3.7.0) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* December 2016
*
* .. Scalar Arguments ..
REAL CSL, CSR, F, G, H, SNL, SNR, SSMAX, SSMIN
* ..
*
* =====================================================================
*
* .. Parameters ..
REAL ZERO
PARAMETER ( ZERO = 0.0E0 )
REAL HALF
PARAMETER ( HALF = 0.5E0 )
REAL ONE
PARAMETER ( ONE = 1.0E0 )
REAL TWO
PARAMETER ( TWO = 2.0E0 )
REAL FOUR
PARAMETER ( FOUR = 4.0E0 )
* ..
* .. Local Scalars ..
LOGICAL GASMAL, SWAP
INTEGER PMAX
REAL A, CLT, CRT, D, FA, FT, GA, GT, HA, HT, L, M,
$ MM, R, S, SLT, SRT, T, TEMP, TSIGN, TT
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, SIGN, SQRT
* ..
* .. External Functions ..
REAL SLAMCH
EXTERNAL SLAMCH
* ..
* .. Executable Statements ..
*
FT = F
FA = ABS( FT )
HT = H
HA = ABS( H )
*
* PMAX points to the maximum absolute element of matrix
* PMAX = 1 if F largest in absolute values
* PMAX = 2 if G largest in absolute values
* PMAX = 3 if H largest in absolute values
*
PMAX = 1
SWAP = ( HA.GT.FA )
IF( SWAP ) THEN
PMAX = 3
TEMP = FT
FT = HT
HT = TEMP
TEMP = FA
FA = HA
HA = TEMP
*
* Now FA .ge. HA
*
END IF
GT = G
GA = ABS( GT )
IF( GA.EQ.ZERO ) THEN
*
* Diagonal matrix
*
SSMIN = HA
SSMAX = FA
CLT = ONE
CRT = ONE
SLT = ZERO
SRT = ZERO
ELSE
GASMAL = .TRUE.
IF( GA.GT.FA ) THEN
PMAX = 2
IF( ( FA / GA ).LT.SLAMCH( 'EPS' ) ) THEN
*
* Case of very large GA
*
GASMAL = .FALSE.
SSMAX = GA
IF( HA.GT.ONE ) THEN
SSMIN = FA / ( GA / HA )
ELSE
SSMIN = ( FA / GA )*HA
END IF
CLT = ONE
SLT = HT / GT
SRT = ONE
CRT = FT / GT
END IF
END IF
IF( GASMAL ) THEN
*
* Normal case
*
D = FA - HA
IF( D.EQ.FA ) THEN
*
* Copes with infinite F or H
*
L = ONE
ELSE
L = D / FA
END IF
*
* Note that 0 .le. L .le. 1
*
M = GT / FT
*
* Note that abs(M) .le. 1/macheps
*
T = TWO - L
*
* Note that T .ge. 1
*
MM = M*M
TT = T*T
S = SQRT( TT+MM )
*
* Note that 1 .le. S .le. 1 + 1/macheps
*
IF( L.EQ.ZERO ) THEN
R = ABS( M )
ELSE
R = SQRT( L*L+MM )
END IF
*
* Note that 0 .le. R .le. 1 + 1/macheps
*
A = HALF*( S+R )
*
* Note that 1 .le. A .le. 1 + abs(M)
*
SSMIN = HA / A
SSMAX = FA*A
IF( MM.EQ.ZERO ) THEN
*
* Note that M is very tiny
*
IF( L.EQ.ZERO ) THEN
T = SIGN( TWO, FT )*SIGN( ONE, GT )
ELSE
T = GT / SIGN( D, FT ) + M / T
END IF
ELSE
T = ( M / ( S+T )+M / ( R+L ) )*( ONE+A )
END IF
L = SQRT( T*T+FOUR )
CRT = TWO / L
SRT = T / L
CLT = ( CRT+SRT*M ) / A
SLT = ( HT / FT )*SRT / A
END IF
END IF
IF( SWAP ) THEN
CSL = SRT
SNL = CRT
CSR = SLT
SNR = CLT
ELSE
CSL = CLT
SNL = SLT
CSR = CRT
SNR = SRT
END IF
*
* Correct signs of SSMAX and SSMIN
*
IF( PMAX.EQ.1 )
$ TSIGN = SIGN( ONE, CSR )*SIGN( ONE, CSL )*SIGN( ONE, F )
IF( PMAX.EQ.2 )
$ TSIGN = SIGN( ONE, SNR )*SIGN( ONE, CSL )*SIGN( ONE, G )
IF( PMAX.EQ.3 )
$ TSIGN = SIGN( ONE, SNR )*SIGN( ONE, SNL )*SIGN( ONE, H )
SSMAX = SIGN( SSMAX, TSIGN )
SSMIN = SIGN( SSMIN, TSIGN*SIGN( ONE, F )*SIGN( ONE, H ) )
RETURN
*
* End of SLASV2
*
END
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