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SUBROUTINE SLAED6( KNITER, ORGATI, RHO, D, Z, FINIT, TAU, INFO )
*
* -- LAPACK routine (version 3.0) --
* Univ. of Tennessee, Oak Ridge National Lab, Argonne National Lab,
* Courant Institute, NAG Ltd., and Rice University
* June 30, 1999
*
* .. Scalar Arguments ..
LOGICAL ORGATI
INTEGER INFO, KNITER
REAL FINIT, RHO, TAU
* ..
* .. Array Arguments ..
REAL D( 3 ), Z( 3 )
* ..
*
* Purpose
* =======
*
* SLAED6 computes the positive or negative root (closest to the origin)
* of
* z(1) z(2) z(3)
* f(x) = rho + --------- + ---------- + ---------
* d(1)-x d(2)-x d(3)-x
*
* It is assumed that
*
* if ORGATI = .true. the root is between d(2) and d(3);
* otherwise it is between d(1) and d(2)
*
* This routine will be called by SLAED4 when necessary. In most cases,
* the root sought is the smallest in magnitude, though it might not be
* in some extremely rare situations.
*
* Arguments
* =========
*
* KNITER (input) INTEGER
* Refer to SLAED4 for its significance.
*
* ORGATI (input) LOGICAL
* If ORGATI is true, the needed root is between d(2) and
* d(3); otherwise it is between d(1) and d(2). See
* SLAED4 for further details.
*
* RHO (input) REAL
* Refer to the equation f(x) above.
*
* D (input) REAL array, dimension (3)
* D satisfies d(1) < d(2) < d(3).
*
* Z (input) REAL array, dimension (3)
* Each of the elements in z must be positive.
*
* FINIT (input) REAL
* The value of f at 0. It is more accurate than the one
* evaluated inside this routine (if someone wants to do
* so).
*
* TAU (output) REAL
* The root of the equation f(x).
*
* INFO (output) INTEGER
* = 0: successful exit
* > 0: if INFO = 1, failure to converge
*
* Further Details
* ===============
*
* Based on contributions by
* Ren-Cang Li, Computer Science Division, University of California
* at Berkeley, USA
*
* =====================================================================
*
* .. Parameters ..
INTEGER MAXIT
PARAMETER ( MAXIT = 20 )
REAL ZERO, ONE, TWO, THREE, FOUR, EIGHT
PARAMETER ( ZERO = 0.0E0, ONE = 1.0E0, TWO = 2.0E0,
$ THREE = 3.0E0, FOUR = 4.0E0, EIGHT = 8.0E0 )
* ..
* .. External Functions ..
REAL SLAMCH
EXTERNAL SLAMCH
* ..
* .. Local Arrays ..
REAL DSCALE( 3 ), ZSCALE( 3 )
* ..
* .. Local Scalars ..
LOGICAL FIRST, SCALE
INTEGER I, ITER, NITER
REAL A, B, BASE, C, DDF, DF, EPS, ERRETM, ETA, F,
$ FC, SCLFAC, SCLINV, SMALL1, SMALL2, SMINV1,
$ SMINV2, TEMP, TEMP1, TEMP2, TEMP3, TEMP4
* ..
* .. Save statement ..
SAVE FIRST, SMALL1, SMINV1, SMALL2, SMINV2, EPS
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, INT, LOG, MAX, MIN, SQRT
* ..
* .. Data statements ..
DATA FIRST / .TRUE. /
* ..
* .. Executable Statements ..
*
INFO = 0
*
NITER = 1
TAU = ZERO
IF( KNITER.EQ.2 ) THEN
IF( ORGATI ) THEN
TEMP = ( D( 3 )-D( 2 ) ) / TWO
C = RHO + Z( 1 ) / ( ( D( 1 )-D( 2 ) )-TEMP )
A = C*( D( 2 )+D( 3 ) ) + Z( 2 ) + Z( 3 )
B = C*D( 2 )*D( 3 ) + Z( 2 )*D( 3 ) + Z( 3 )*D( 2 )
ELSE
TEMP = ( D( 1 )-D( 2 ) ) / TWO
C = RHO + Z( 3 ) / ( ( D( 3 )-D( 2 ) )-TEMP )
A = C*( D( 1 )+D( 2 ) ) + Z( 1 ) + Z( 2 )
B = C*D( 1 )*D( 2 ) + Z( 1 )*D( 2 ) + Z( 2 )*D( 1 )
END IF
TEMP = MAX( ABS( A ), ABS( B ), ABS( C ) )
A = A / TEMP
B = B / TEMP
C = C / TEMP
IF( C.EQ.ZERO ) THEN
TAU = B / A
ELSE IF( A.LE.ZERO ) THEN
TAU = ( A-SQRT( ABS( A*A-FOUR*B*C ) ) ) / ( TWO*C )
ELSE
TAU = TWO*B / ( A+SQRT( ABS( A*A-FOUR*B*C ) ) )
END IF
TEMP = RHO + Z( 1 ) / ( D( 1 )-TAU ) +
$ Z( 2 ) / ( D( 2 )-TAU ) + Z( 3 ) / ( D( 3 )-TAU )
IF( ABS( FINIT ).LE.ABS( TEMP ) )
$ TAU = ZERO
END IF
*
* On first call to routine, get machine parameters for
* possible scaling to avoid overflow
*
IF( FIRST ) THEN
EPS = SLAMCH( 'Epsilon' )
BASE = SLAMCH( 'Base' )
SMALL1 = BASE**( INT( LOG( SLAMCH( 'SafMin' ) ) / LOG( BASE ) /
$ THREE ) )
SMINV1 = ONE / SMALL1
SMALL2 = SMALL1*SMALL1
SMINV2 = SMINV1*SMINV1
FIRST = .FALSE.
END IF
*
* Determine if scaling of inputs necessary to avoid overflow
* when computing 1/TEMP**3
*
IF( ORGATI ) THEN
TEMP = MIN( ABS( D( 2 )-TAU ), ABS( D( 3 )-TAU ) )
ELSE
TEMP = MIN( ABS( D( 1 )-TAU ), ABS( D( 2 )-TAU ) )
END IF
SCALE = .FALSE.
IF( TEMP.LE.SMALL1 ) THEN
SCALE = .TRUE.
IF( TEMP.LE.SMALL2 ) THEN
*
* Scale up by power of radix nearest 1/SAFMIN**(2/3)
*
SCLFAC = SMINV2
SCLINV = SMALL2
ELSE
*
* Scale up by power of radix nearest 1/SAFMIN**(1/3)
*
SCLFAC = SMINV1
SCLINV = SMALL1
END IF
*
* Scaling up safe because D, Z, TAU scaled elsewhere to be O(1)
*
DO 10 I = 1, 3
DSCALE( I ) = D( I )*SCLFAC
ZSCALE( I ) = Z( I )*SCLFAC
10 CONTINUE
TAU = TAU*SCLFAC
ELSE
*
* Copy D and Z to DSCALE and ZSCALE
*
DO 20 I = 1, 3
DSCALE( I ) = D( I )
ZSCALE( I ) = Z( I )
20 CONTINUE
END IF
*
FC = ZERO
DF = ZERO
DDF = ZERO
DO 30 I = 1, 3
TEMP = ONE / ( DSCALE( I )-TAU )
TEMP1 = ZSCALE( I )*TEMP
TEMP2 = TEMP1*TEMP
TEMP3 = TEMP2*TEMP
FC = FC + TEMP1 / DSCALE( I )
DF = DF + TEMP2
DDF = DDF + TEMP3
30 CONTINUE
F = FINIT + TAU*FC
*
IF( ABS( F ).LE.ZERO )
$ GO TO 60
*
* Iteration begins
*
* It is not hard to see that
*
* 1) Iterations will go up monotonically
* if FINIT < 0;
*
* 2) Iterations will go down monotonically
* if FINIT > 0.
*
ITER = NITER + 1
*
DO 50 NITER = ITER, MAXIT
*
IF( ORGATI ) THEN
TEMP1 = DSCALE( 2 ) - TAU
TEMP2 = DSCALE( 3 ) - TAU
ELSE
TEMP1 = DSCALE( 1 ) - TAU
TEMP2 = DSCALE( 2 ) - TAU
END IF
A = ( TEMP1+TEMP2 )*F - TEMP1*TEMP2*DF
B = TEMP1*TEMP2*F
C = F - ( TEMP1+TEMP2 )*DF + TEMP1*TEMP2*DDF
TEMP = MAX( ABS( A ), ABS( B ), ABS( C ) )
A = A / TEMP
B = B / TEMP
C = C / TEMP
IF( C.EQ.ZERO ) THEN
ETA = B / A
ELSE IF( A.LE.ZERO ) THEN
ETA = ( A-SQRT( ABS( A*A-FOUR*B*C ) ) ) / ( TWO*C )
ELSE
ETA = TWO*B / ( A+SQRT( ABS( A*A-FOUR*B*C ) ) )
END IF
IF( F*ETA.GE.ZERO ) THEN
ETA = -F / DF
END IF
*
TEMP = ETA + TAU
IF( ORGATI ) THEN
IF( ETA.GT.ZERO .AND. TEMP.GE.DSCALE( 3 ) )
$ ETA = ( DSCALE( 3 )-TAU ) / TWO
IF( ETA.LT.ZERO .AND. TEMP.LE.DSCALE( 2 ) )
$ ETA = ( DSCALE( 2 )-TAU ) / TWO
ELSE
IF( ETA.GT.ZERO .AND. TEMP.GE.DSCALE( 2 ) )
$ ETA = ( DSCALE( 2 )-TAU ) / TWO
IF( ETA.LT.ZERO .AND. TEMP.LE.DSCALE( 1 ) )
$ ETA = ( DSCALE( 1 )-TAU ) / TWO
END IF
TAU = TAU + ETA
*
FC = ZERO
ERRETM = ZERO
DF = ZERO
DDF = ZERO
DO 40 I = 1, 3
TEMP = ONE / ( DSCALE( I )-TAU )
TEMP1 = ZSCALE( I )*TEMP
TEMP2 = TEMP1*TEMP
TEMP3 = TEMP2*TEMP
TEMP4 = TEMP1 / DSCALE( I )
FC = FC + TEMP4
ERRETM = ERRETM + ABS( TEMP4 )
DF = DF + TEMP2
DDF = DDF + TEMP3
40 CONTINUE
F = FINIT + TAU*FC
ERRETM = EIGHT*( ABS( FINIT )+ABS( TAU )*ERRETM ) +
$ ABS( TAU )*DF
IF( ABS( F ).LE.EPS*ERRETM )
$ GO TO 60
50 CONTINUE
INFO = 1
60 CONTINUE
*
* Undo scaling
*
IF( SCALE )
$ TAU = TAU*SCLINV
RETURN
*
* End of SLAED6
*
END
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