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SUBROUTINE SLAED6( KNITER, ORGATI, RHO, D, Z, FINIT, TAU, INFO )
* * -- LAPACK routine (version 3.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * February 2007 * * .. 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 * =============== * * 30/06/99: Based on contributions by * Ren-Cang Li, Computer Science Division, University of California * at Berkeley, USA * * 10/02/03: This version has a few statements commented out for thread safety * (machine parameters are computed on each entry). SJH. * * 05/10/06: Modified from a new version of Ren-Cang Li, use * Gragg-Thornton-Warner cubic convergent scheme for better stability. * * ===================================================================== * * .. Parameters .. INTEGER MAXIT PARAMETER ( MAXIT = 40 ) 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 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, $ LBD, UBD * .. * .. Intrinsic Functions .. INTRINSIC ABS, INT, LOG, MAX, MIN, SQRT * .. * .. Executable Statements .. * INFO = 0 * IF( ORGATI ) THEN LBD = D(2) UBD = D(3) ELSE LBD = D(1) UBD = D(2) END IF IF( FINIT .LT. ZERO )THEN LBD = ZERO ELSE UBD = ZERO END IF * 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 IF( TAU .LT. LBD .OR. TAU .GT. UBD ) $ TAU = ( LBD+UBD )/TWO IF( D(1).EQ.TAU .OR. D(2).EQ.TAU .OR. D(3).EQ.TAU ) THEN TAU = ZERO ELSE TEMP = FINIT + TAU*Z(1)/( D(1)*( D( 1 )-TAU ) ) + $ TAU*Z(2)/( D(2)*( D( 2 )-TAU ) ) + $ TAU*Z(3)/( D(3)*( D( 3 )-TAU ) ) IF( TEMP .LE. ZERO )THEN LBD = TAU ELSE UBD = TAU END IF IF( ABS( FINIT ).LE.ABS( TEMP ) ) $ TAU = ZERO END IF END IF * * get machine parameters for possible scaling to avoid overflow * * modified by Sven: parameters SMALL1, SMINV1, SMALL2, * SMINV2, EPS are not SAVEd anymore between one call to the * others but recomputed at each call * EPS = SLAMCH( 'Epsilon' ) BASE = SLAMCH( 'Base' ) SMALL1 = BASE**( INT( LOG( SLAMCH( 'SafMin' ) ) / LOG( BASE ) / $ THREE ) ) SMINV1 = ONE / SMALL1 SMALL2 = SMALL1*SMALL1 SMINV2 = SMINV1*SMINV1 * * 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 LBD = LBD*SCLFAC UBD = UBD*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 IF( F .LE. ZERO )THEN LBD = TAU ELSE UBD = TAU END IF * * Iteration begins -- Use Gragg-Thornton-Warner cubic convergent * scheme * * 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 * TAU = TAU + ETA IF( TAU .LT. LBD .OR. TAU .GT. UBD ) $ TAU = ( LBD + UBD )/TWO * 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 IF( F .LE. ZERO )THEN LBD = TAU ELSE UBD = TAU END IF 50 CONTINUE INFO = 1 60 CONTINUE * * Undo scaling * IF( SCALE ) $ TAU = TAU*SCLINV RETURN * * End of SLAED6 * END |