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SUBROUTINE DLAS2( F, G, H, SSMIN, SSMAX )
*
* -- LAPACK auxiliary routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
DOUBLE PRECISION F, G, H, SSMAX, SSMIN
* ..
*
* Purpose
* =======
*
* DLAS2 computes the singular values of the 2-by-2 matrix
* [ F G ]
* [ 0 H ].
* On return, SSMIN is the smaller singular value and SSMAX is the
* larger singular value.
*
* Arguments
* =========
*
* F (input) DOUBLE PRECISION
* The (1,1) element of the 2-by-2 matrix.
*
* G (input) DOUBLE PRECISION
* The (1,2) element of the 2-by-2 matrix.
*
* H (input) DOUBLE PRECISION
* The (2,2) element of the 2-by-2 matrix.
*
* SSMIN (output) DOUBLE PRECISION
* The smaller singular value.
*
* SSMAX (output) DOUBLE PRECISION
* The larger singular value.
*
* Further Details
* ===============
*
* Barring over/underflow, all output quantities are correct to within
* a few units in the last place (ulps), even in the absence of a guard
* digit in addition/subtraction.
*
* 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.
*
* ====================================================================
*
* .. Parameters ..
DOUBLE PRECISION ZERO
PARAMETER ( ZERO = 0.0D0 )
DOUBLE PRECISION ONE
PARAMETER ( ONE = 1.0D0 )
DOUBLE PRECISION TWO
PARAMETER ( TWO = 2.0D0 )
* ..
* .. Local Scalars ..
DOUBLE PRECISION AS, AT, AU, C, FA, FHMN, FHMX, GA, HA
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, MAX, MIN, SQRT
* ..
* .. Executable Statements ..
*
FA = ABS( F )
GA = ABS( G )
HA = ABS( H )
FHMN = MIN( FA, HA )
FHMX = MAX( FA, HA )
IF( FHMN.EQ.ZERO ) THEN
SSMIN = ZERO
IF( FHMX.EQ.ZERO ) THEN
SSMAX = GA
ELSE
SSMAX = MAX( FHMX, GA )*SQRT( ONE+
$ ( MIN( FHMX, GA ) / MAX( FHMX, GA ) )**2 )
END IF
ELSE
IF( GA.LT.FHMX ) THEN
AS = ONE + FHMN / FHMX
AT = ( FHMX-FHMN ) / FHMX
AU = ( GA / FHMX )**2
C = TWO / ( SQRT( AS*AS+AU )+SQRT( AT*AT+AU ) )
SSMIN = FHMN*C
SSMAX = FHMX / C
ELSE
AU = FHMX / GA
IF( AU.EQ.ZERO ) THEN
*
* Avoid possible harmful underflow if exponent range
* asymmetric (true SSMIN may not underflow even if
* AU underflows)
*
SSMIN = ( FHMN*FHMX ) / GA
SSMAX = GA
ELSE
AS = ONE + FHMN / FHMX
AT = ( FHMX-FHMN ) / FHMX
C = ONE / ( SQRT( ONE+( AS*AU )**2 )+
$ SQRT( ONE+( AT*AU )**2 ) )
SSMIN = ( FHMN*C )*AU
SSMIN = SSMIN + SSMIN
SSMAX = GA / ( C+C )
END IF
END IF
END IF
RETURN
*
* End of DLAS2
*
END
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