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SUBROUTINE ZLARFG( N, ALPHA, X, INCX, TAU )
*
* -- LAPACK auxiliary routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
INTEGER INCX, N
COMPLEX*16 ALPHA, TAU
* ..
* .. Array Arguments ..
COMPLEX*16 X( * )
* ..
*
* Purpose
* =======
*
* ZLARFG generates a complex elementary reflector H of order n, such
* that
*
* H' * ( alpha ) = ( beta ), H' * H = I.
* ( x ) ( 0 )
*
* where alpha and beta are scalars, with beta real, and x is an
* (n-1)-element complex vector. H is represented in the form
*
* H = I - tau * ( 1 ) * ( 1 v' ) ,
* ( v )
*
* where tau is a complex scalar and v is a complex (n-1)-element
* vector. Note that H is not hermitian.
*
* If the elements of x are all zero and alpha is real, then tau = 0
* and H is taken to be the unit matrix.
*
* Otherwise 1 <= real(tau) <= 2 and abs(tau-1) <= 1 .
*
* Arguments
* =========
*
* N (input) INTEGER
* The order of the elementary reflector.
*
* ALPHA (input/output) COMPLEX*16
* On entry, the value alpha.
* On exit, it is overwritten with the value beta.
*
* X (input/output) COMPLEX*16 array, dimension
* (1+(N-2)*abs(INCX))
* On entry, the vector x.
* On exit, it is overwritten with the vector v.
*
* INCX (input) INTEGER
* The increment between elements of X. INCX > 0.
*
* TAU (output) COMPLEX*16
* The value tau.
*
* =====================================================================
*
* .. Parameters ..
DOUBLE PRECISION ONE, ZERO
PARAMETER ( ONE = 1.0D+0, ZERO = 0.0D+0 )
* ..
* .. Local Scalars ..
INTEGER J, KNT
DOUBLE PRECISION ALPHI, ALPHR, BETA, RSAFMN, SAFMIN, XNORM
* ..
* .. External Functions ..
DOUBLE PRECISION DLAMCH, DLAPY3, DZNRM2
COMPLEX*16 ZLADIV
EXTERNAL DLAMCH, DLAPY3, DZNRM2, ZLADIV
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, DBLE, DCMPLX, DIMAG, SIGN
* ..
* .. External Subroutines ..
EXTERNAL ZDSCAL, ZSCAL
* ..
* .. Executable Statements ..
*
IF( N.LE.0 ) THEN
TAU = ZERO
RETURN
END IF
*
XNORM = DZNRM2( N-1, X, INCX )
ALPHR = DBLE( ALPHA )
ALPHI = DIMAG( ALPHA )
*
IF( XNORM.EQ.ZERO .AND. ALPHI.EQ.ZERO ) THEN
*
* H = I
*
TAU = ZERO
ELSE
*
* general case
*
BETA = -SIGN( DLAPY3( ALPHR, ALPHI, XNORM ), ALPHR )
SAFMIN = DLAMCH( 'S' ) / DLAMCH( 'E' )
RSAFMN = ONE / SAFMIN
*
IF( ABS( BETA ).LT.SAFMIN ) THEN
*
* XNORM, BETA may be inaccurate; scale X and recompute them
*
KNT = 0
10 CONTINUE
KNT = KNT + 1
CALL ZDSCAL( N-1, RSAFMN, X, INCX )
BETA = BETA*RSAFMN
ALPHI = ALPHI*RSAFMN
ALPHR = ALPHR*RSAFMN
IF( ABS( BETA ).LT.SAFMIN )
$ GO TO 10
*
* New BETA is at most 1, at least SAFMIN
*
XNORM = DZNRM2( N-1, X, INCX )
ALPHA = DCMPLX( ALPHR, ALPHI )
BETA = -SIGN( DLAPY3( ALPHR, ALPHI, XNORM ), ALPHR )
TAU = DCMPLX( ( BETA-ALPHR ) / BETA, -ALPHI / BETA )
ALPHA = ZLADIV( DCMPLX( ONE ), ALPHA-BETA )
CALL ZSCAL( N-1, ALPHA, X, INCX )
*
* If ALPHA is subnormal, it may lose relative accuracy
*
ALPHA = BETA
DO 20 J = 1, KNT
ALPHA = ALPHA*SAFMIN
20 CONTINUE
ELSE
TAU = DCMPLX( ( BETA-ALPHR ) / BETA, -ALPHI / BETA )
ALPHA = ZLADIV( DCMPLX( ONE ), ALPHA-BETA )
CALL ZSCAL( N-1, ALPHA, X, INCX )
ALPHA = BETA
END IF
END IF
*
RETURN
*
* End of ZLARFG
*
END
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