141 lines
3.7 KiB
FortranFixed
141 lines
3.7 KiB
FortranFixed
SUBROUTINE CLARFG( N, ALPHA, X, INCX, TAU )
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*
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* -- LAPACK auxiliary routine (version 3.1) --
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* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
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* November 2006
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*
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* .. Scalar Arguments ..
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INTEGER INCX, N
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COMPLEX ALPHA, TAU
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* ..
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* .. Array Arguments ..
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COMPLEX X( * )
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* ..
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*
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* Purpose
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* =======
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*
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* CLARFG generates a complex elementary reflector H of order n, such
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* that
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*
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* H' * ( alpha ) = ( beta ), H' * H = I.
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* ( x ) ( 0 )
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*
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* where alpha and beta are scalars, with beta real, and x is an
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* (n-1)-element complex vector. H is represented in the form
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*
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* H = I - tau * ( 1 ) * ( 1 v' ) ,
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* ( v )
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*
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* where tau is a complex scalar and v is a complex (n-1)-element
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* vector. Note that H is not hermitian.
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*
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* If the elements of x are all zero and alpha is real, then tau = 0
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* and H is taken to be the unit matrix.
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*
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* Otherwise 1 <= real(tau) <= 2 and abs(tau-1) <= 1 .
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*
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* Arguments
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* =========
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*
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* N (input) INTEGER
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* The order of the elementary reflector.
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*
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* ALPHA (input/output) COMPLEX
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* On entry, the value alpha.
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* On exit, it is overwritten with the value beta.
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*
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* X (input/output) COMPLEX array, dimension
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* (1+(N-2)*abs(INCX))
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* On entry, the vector x.
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* On exit, it is overwritten with the vector v.
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*
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* INCX (input) INTEGER
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* The increment between elements of X. INCX > 0.
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*
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* TAU (output) COMPLEX
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* The value tau.
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*
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* =====================================================================
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*
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* .. Parameters ..
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REAL ONE, ZERO
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PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 )
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* ..
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* .. Local Scalars ..
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INTEGER J, KNT
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REAL ALPHI, ALPHR, BETA, RSAFMN, SAFMIN, XNORM
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* ..
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* .. External Functions ..
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REAL SCNRM2, SLAMCH, SLAPY3
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COMPLEX CLADIV
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EXTERNAL SCNRM2, SLAMCH, SLAPY3, CLADIV
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS, AIMAG, CMPLX, REAL, SIGN
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* ..
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* .. External Subroutines ..
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EXTERNAL CSCAL, CSSCAL
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* ..
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* .. Executable Statements ..
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*
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IF( N.LE.0 ) THEN
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TAU = ZERO
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RETURN
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END IF
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*
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XNORM = SCNRM2( N-1, X, INCX )
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ALPHR = REAL( ALPHA )
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ALPHI = AIMAG( ALPHA )
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*
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IF( XNORM.EQ.ZERO .AND. ALPHI.EQ.ZERO ) THEN
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*
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* H = I
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*
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TAU = ZERO
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ELSE
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*
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* general case
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*
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BETA = -SIGN( SLAPY3( ALPHR, ALPHI, XNORM ), ALPHR )
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SAFMIN = SLAMCH( 'S' ) / SLAMCH( 'E' )
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RSAFMN = ONE / SAFMIN
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*
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KNT = 0
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IF( ABS( BETA ).LT.SAFMIN ) THEN
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*
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* XNORM, BETA may be inaccurate; scale X and recompute them
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*
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10 CONTINUE
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KNT = KNT + 1
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CALL CSSCAL( N-1, RSAFMN, X, INCX )
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BETA = BETA*RSAFMN
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ALPHI = ALPHI*RSAFMN
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ALPHR = ALPHR*RSAFMN
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IF( ABS( BETA ).LT.SAFMIN )
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$ GO TO 10
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*
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* New BETA is at most 1, at least SAFMIN
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*
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XNORM = SCNRM2( N-1, X, INCX )
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ALPHA = CMPLX( ALPHR, ALPHI )
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BETA = -SIGN( SLAPY3( ALPHR, ALPHI, XNORM ), ALPHR )
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END IF
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TAU = CMPLX( ( BETA-ALPHR ) / BETA, -ALPHI / BETA )
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ALPHA = CLADIV( CMPLX( ONE ), ALPHA-BETA )
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CALL CSCAL( N-1, ALPHA, X, INCX )
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*
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* If ALPHA is subnormal, it may lose relative accuracy
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*
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DO 20 J = 1, KNT
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BETA = BETA*SAFMIN
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20 CONTINUE
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ALPHA = BETA
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END IF
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*
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RETURN
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*
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* End of CLARFG
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*
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END
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