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SUBROUTINE ZLAIC1( JOB, J, X, SEST, W, GAMMA, SESTPR, S, C )
*
* -- LAPACK auxiliary routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
INTEGER J, JOB
DOUBLE PRECISION SEST, SESTPR
COMPLEX*16 C, GAMMA, S
* ..
* .. Array Arguments ..
COMPLEX*16 W( J ), X( J )
* ..
*
* Purpose
* =======
*
* ZLAIC1 applies one step of incremental condition estimation in
* its simplest version:
*
* Let x, twonorm(x) = 1, be an approximate singular vector of an j-by-j
* lower triangular matrix L, such that
* twonorm(L*x) = sest
* Then ZLAIC1 computes sestpr, s, c such that
* the vector
* [ s*x ]
* xhat = [ c ]
* is an approximate singular vector of
* [ L 0 ]
* Lhat = [ w' gamma ]
* in the sense that
* twonorm(Lhat*xhat) = sestpr.
*
* Depending on JOB, an estimate for the largest or smallest singular
* value is computed.
*
* Note that [s c]' and sestpr**2 is an eigenpair of the system
*
* diag(sest*sest, 0) + [alpha gamma] * [ conjg(alpha) ]
* [ conjg(gamma) ]
*
* where alpha = conjg(x)'*w.
*
* Arguments
* =========
*
* JOB (input) INTEGER
* = 1: an estimate for the largest singular value is computed.
* = 2: an estimate for the smallest singular value is computed.
*
* J (input) INTEGER
* Length of X and W
*
* X (input) COMPLEX*16 array, dimension (J)
* The j-vector x.
*
* SEST (input) DOUBLE PRECISION
* Estimated singular value of j by j matrix L
*
* W (input) COMPLEX*16 array, dimension (J)
* The j-vector w.
*
* GAMMA (input) COMPLEX*16
* The diagonal element gamma.
*
* SESTPR (output) DOUBLE PRECISION
* Estimated singular value of (j+1) by (j+1) matrix Lhat.
*
* S (output) COMPLEX*16
* Sine needed in forming xhat.
*
* C (output) COMPLEX*16
* Cosine needed in forming xhat.
*
* =====================================================================
*
* .. Parameters ..
DOUBLE PRECISION ZERO, ONE, TWO
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0, TWO = 2.0D0 )
DOUBLE PRECISION HALF, FOUR
PARAMETER ( HALF = 0.5D0, FOUR = 4.0D0 )
* ..
* .. Local Scalars ..
DOUBLE PRECISION ABSALP, ABSEST, ABSGAM, B, EPS, NORMA, S1, S2,
$ SCL, T, TEST, TMP, ZETA1, ZETA2
COMPLEX*16 ALPHA, COSINE, SINE
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, DCONJG, MAX, SQRT
* ..
* .. External Functions ..
DOUBLE PRECISION DLAMCH
COMPLEX*16 ZDOTC
EXTERNAL DLAMCH, ZDOTC
* ..
* .. Executable Statements ..
*
EPS = DLAMCH( 'Epsilon' )
ALPHA = ZDOTC( J, X, 1, W, 1 )
*
ABSALP = ABS( ALPHA )
ABSGAM = ABS( GAMMA )
ABSEST = ABS( SEST )
*
IF( JOB.EQ.1 ) THEN
*
* Estimating largest singular value
*
* special cases
*
IF( SEST.EQ.ZERO ) THEN
S1 = MAX( ABSGAM, ABSALP )
IF( S1.EQ.ZERO ) THEN
S = ZERO
C = ONE
SESTPR = ZERO
ELSE
S = ALPHA / S1
C = GAMMA / S1
TMP = SQRT( S*DCONJG( S )+C*DCONJG( C ) )
S = S / TMP
C = C / TMP
SESTPR = S1*TMP
END IF
RETURN
ELSE IF( ABSGAM.LE.EPS*ABSEST ) THEN
S = ONE
C = ZERO
TMP = MAX( ABSEST, ABSALP )
S1 = ABSEST / TMP
S2 = ABSALP / TMP
SESTPR = TMP*SQRT( S1*S1+S2*S2 )
RETURN
ELSE IF( ABSALP.LE.EPS*ABSEST ) THEN
S1 = ABSGAM
S2 = ABSEST
IF( S1.LE.S2 ) THEN
S = ONE
C = ZERO
SESTPR = S2
ELSE
S = ZERO
C = ONE
SESTPR = S1
END IF
RETURN
ELSE IF( ABSEST.LE.EPS*ABSALP .OR. ABSEST.LE.EPS*ABSGAM ) THEN
S1 = ABSGAM
S2 = ABSALP
IF( S1.LE.S2 ) THEN
TMP = S1 / S2
SCL = SQRT( ONE+TMP*TMP )
SESTPR = S2*SCL
S = ( ALPHA / S2 ) / SCL
C = ( GAMMA / S2 ) / SCL
ELSE
TMP = S2 / S1
SCL = SQRT( ONE+TMP*TMP )
SESTPR = S1*SCL
S = ( ALPHA / S1 ) / SCL
C = ( GAMMA / S1 ) / SCL
END IF
RETURN
ELSE
*
* normal case
*
ZETA1 = ABSALP / ABSEST
ZETA2 = ABSGAM / ABSEST
*
B = ( ONE-ZETA1*ZETA1-ZETA2*ZETA2 )*HALF
C = ZETA1*ZETA1
IF( B.GT.ZERO ) THEN
T = C / ( B+SQRT( B*B+C ) )
ELSE
T = SQRT( B*B+C ) - B
END IF
*
SINE = -( ALPHA / ABSEST ) / T
COSINE = -( GAMMA / ABSEST ) / ( ONE+T )
TMP = SQRT( SINE*DCONJG( SINE )+COSINE*DCONJG( COSINE ) )
S = SINE / TMP
C = COSINE / TMP
SESTPR = SQRT( T+ONE )*ABSEST
RETURN
END IF
*
ELSE IF( JOB.EQ.2 ) THEN
*
* Estimating smallest singular value
*
* special cases
*
IF( SEST.EQ.ZERO ) THEN
SESTPR = ZERO
IF( MAX( ABSGAM, ABSALP ).EQ.ZERO ) THEN
SINE = ONE
COSINE = ZERO
ELSE
SINE = -DCONJG( GAMMA )
COSINE = DCONJG( ALPHA )
END IF
S1 = MAX( ABS( SINE ), ABS( COSINE ) )
S = SINE / S1
C = COSINE / S1
TMP = SQRT( S*DCONJG( S )+C*DCONJG( C ) )
S = S / TMP
C = C / TMP
RETURN
ELSE IF( ABSGAM.LE.EPS*ABSEST ) THEN
S = ZERO
C = ONE
SESTPR = ABSGAM
RETURN
ELSE IF( ABSALP.LE.EPS*ABSEST ) THEN
S1 = ABSGAM
S2 = ABSEST
IF( S1.LE.S2 ) THEN
S = ZERO
C = ONE
SESTPR = S1
ELSE
S = ONE
C = ZERO
SESTPR = S2
END IF
RETURN
ELSE IF( ABSEST.LE.EPS*ABSALP .OR. ABSEST.LE.EPS*ABSGAM ) THEN
S1 = ABSGAM
S2 = ABSALP
IF( S1.LE.S2 ) THEN
TMP = S1 / S2
SCL = SQRT( ONE+TMP*TMP )
SESTPR = ABSEST*( TMP / SCL )
S = -( DCONJG( GAMMA ) / S2 ) / SCL
C = ( DCONJG( ALPHA ) / S2 ) / SCL
ELSE
TMP = S2 / S1
SCL = SQRT( ONE+TMP*TMP )
SESTPR = ABSEST / SCL
S = -( DCONJG( GAMMA ) / S1 ) / SCL
C = ( DCONJG( ALPHA ) / S1 ) / SCL
END IF
RETURN
ELSE
*
* normal case
*
ZETA1 = ABSALP / ABSEST
ZETA2 = ABSGAM / ABSEST
*
NORMA = MAX( ONE+ZETA1*ZETA1+ZETA1*ZETA2,
$ ZETA1*ZETA2+ZETA2*ZETA2 )
*
* See if root is closer to zero or to ONE
*
TEST = ONE + TWO*( ZETA1-ZETA2 )*( ZETA1+ZETA2 )
IF( TEST.GE.ZERO ) THEN
*
* root is close to zero, compute directly
*
B = ( ZETA1*ZETA1+ZETA2*ZETA2+ONE )*HALF
C = ZETA2*ZETA2
T = C / ( B+SQRT( ABS( B*B-C ) ) )
SINE = ( ALPHA / ABSEST ) / ( ONE-T )
COSINE = -( GAMMA / ABSEST ) / T
SESTPR = SQRT( T+FOUR*EPS*EPS*NORMA )*ABSEST
ELSE
*
* root is closer to ONE, shift by that amount
*
B = ( ZETA2*ZETA2+ZETA1*ZETA1-ONE )*HALF
C = ZETA1*ZETA1
IF( B.GE.ZERO ) THEN
T = -C / ( B+SQRT( B*B+C ) )
ELSE
T = B - SQRT( B*B+C )
END IF
SINE = -( ALPHA / ABSEST ) / T
COSINE = -( GAMMA / ABSEST ) / ( ONE+T )
SESTPR = SQRT( ONE+T+FOUR*EPS*EPS*NORMA )*ABSEST
END IF
TMP = SQRT( SINE*DCONJG( SINE )+COSINE*DCONJG( COSINE ) )
S = SINE / TMP
C = COSINE / TMP
RETURN
*
END IF
END IF
RETURN
*
* End of ZLAIC1
*
END
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