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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