SUBROUTINE CLARFGP( N, ALPHA, X, INCX, TAU ) * * -- LAPACK auxiliary routine (version 3.2.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * June 2010 * * .. Scalar Arguments .. INTEGER INCX, N COMPLEX ALPHA, TAU * .. * .. Array Arguments .. COMPLEX X( * ) * .. * * Purpose * ======= * * CLARFGP 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, beta is real and non-negative, 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. * * Arguments * ========= * * N (input) INTEGER * The order of the elementary reflector. * * ALPHA (input/output) COMPLEX * On entry, the value alpha. * On exit, it is overwritten with the value beta. * * X (input/output) COMPLEX 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 * The value tau. * * ===================================================================== * * .. Parameters .. REAL TWO, ONE, ZERO PARAMETER ( TWO = 2.0E+0, ONE = 1.0E+0, ZERO = 0.0E+0 ) * .. * .. Local Scalars .. INTEGER J, KNT REAL ALPHI, ALPHR, BETA, BIGNUM, SMLNUM, XNORM COMPLEX SAVEALPHA * .. * .. External Functions .. REAL SCNRM2, SLAMCH, SLAPY3, SLAPY2 COMPLEX CLADIV EXTERNAL SCNRM2, SLAMCH, SLAPY3, SLAPY2, CLADIV * .. * .. Intrinsic Functions .. INTRINSIC ABS, AIMAG, CMPLX, REAL, SIGN * .. * .. External Subroutines .. EXTERNAL CSCAL, CSSCAL * .. * .. Executable Statements .. * IF( N.LE.0 ) THEN TAU = ZERO RETURN END IF * XNORM = SCNRM2( N-1, X, INCX ) ALPHR = REAL( ALPHA ) ALPHI = AIMAG( ALPHA ) * IF( XNORM.EQ.ZERO ) THEN * * H = [1-alpha/abs(alpha) 0; 0 I], sign chosen so ALPHA >= 0. * IF( ALPHI.EQ.ZERO ) THEN IF( ALPHR.GE.ZERO ) THEN * When TAU.eq.ZERO, the vector is special-cased to be * all zeros in the application routines. We do not need * to clear it. TAU = ZERO ELSE * However, the application routines rely on explicit * zero checks when TAU.ne.ZERO, and we must clear X. TAU = TWO DO J = 1, N-1 X( 1 + (J-1)*INCX ) = ZERO END DO ALPHA = -ALPHA END IF ELSE * Only "reflecting" the diagonal entry to be real and non-negative. XNORM = SLAPY2( ALPHR, ALPHI ) TAU = CMPLX( ONE - ALPHR / XNORM, -ALPHI / XNORM ) DO J = 1, N-1 X( 1 + (J-1)*INCX ) = ZERO END DO ALPHA = XNORM END IF ELSE * * general case * BETA = SIGN( SLAPY3( ALPHR, ALPHI, XNORM ), ALPHR ) SMLNUM = SLAMCH( 'S' ) / SLAMCH( 'E' ) BIGNUM = ONE / SMLNUM * KNT = 0 IF( ABS( BETA ).LT.SMLNUM ) THEN * * XNORM, BETA may be inaccurate; scale X and recompute them * 10 CONTINUE KNT = KNT + 1 CALL CSSCAL( N-1, BIGNUM, X, INCX ) BETA = BETA*BIGNUM ALPHI = ALPHI*BIGNUM ALPHR = ALPHR*BIGNUM IF( ABS( BETA ).LT.SMLNUM ) \$ GO TO 10 * * New BETA is at most 1, at least SMLNUM * XNORM = SCNRM2( N-1, X, INCX ) ALPHA = CMPLX( ALPHR, ALPHI ) BETA = SIGN( SLAPY3( ALPHR, ALPHI, XNORM ), ALPHR ) END IF SAVEALPHA = ALPHA ALPHA = ALPHA + BETA IF( BETA.LT.ZERO ) THEN BETA = -BETA TAU = -ALPHA / BETA ELSE ALPHR = ALPHI * (ALPHI/REAL( ALPHA )) ALPHR = ALPHR + XNORM * (XNORM/REAL( ALPHA )) TAU = CMPLX( ALPHR/BETA, -ALPHI/BETA ) ALPHA = CMPLX( -ALPHR, ALPHI ) END IF ALPHA = CLADIV( CMPLX( ONE ), ALPHA ) * IF ( ABS(TAU).LE.SMLNUM ) THEN * * In the case where the computed TAU ends up being a denormalized number, * it loses relative accuracy. This is a BIG problem. Solution: flush TAU * to ZERO (or TWO or whatever makes a nonnegative real number for BETA). * * (Bug report provided by Pat Quillen from MathWorks on Jul 29, 2009.) * (Thanks Pat. Thanks MathWorks.) * ALPHR = REAL( SAVEALPHA ) ALPHI = AIMAG( SAVEALPHA ) IF( ALPHI.EQ.ZERO ) THEN IF( ALPHR.GE.ZERO ) THEN TAU = ZERO ELSE TAU = TWO DO J = 1, N-1 X( 1 + (J-1)*INCX ) = ZERO END DO BETA = -SAVEALPHA END IF ELSE XNORM = SLAPY2( ALPHR, ALPHI ) TAU = CMPLX( ONE - ALPHR / XNORM, -ALPHI / XNORM ) DO J = 1, N-1 X( 1 + (J-1)*INCX ) = ZERO END DO BETA = XNORM END IF * ELSE * * This is the general case. * CALL CSCAL( N-1, ALPHA, X, INCX ) * END IF * * If BETA is subnormal, it may lose relative accuracy * DO 20 J = 1, KNT BETA = BETA*SMLNUM 20 CONTINUE ALPHA = BETA END IF * RETURN * * End of CLARFGP * END