#include "blaswrap.h"
/* dlasd5.f -- translated by f2c (version 20061008).
   You must link the resulting object file with libf2c:
	on Microsoft Windows system, link with libf2c.lib;
	on Linux or Unix systems, link with .../path/to/libf2c.a -lm
	or, if you install libf2c.a in a standard place, with -lf2c -lm
	-- in that order, at the end of the command line, as in
		cc *.o -lf2c -lm
	Source for libf2c is in /netlib/f2c/libf2c.zip, e.g.,

		http://www.netlib.org/f2c/libf2c.zip
*/

#include "f2c.h"

/* Subroutine */ int dlasd5_(integer *i__, doublereal *d__, doublereal *z__, 
	doublereal *delta, doublereal *rho, doublereal *dsigma, doublereal *
	work)
{
    /* System generated locals */
    doublereal d__1;

    /* Builtin functions */
    double sqrt(doublereal);

    /* Local variables */
    static doublereal b, c__, w, del, tau, delsq;


/*  -- LAPACK auxiliary routine (version 3.1) --   
       Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..   
       November 2006   


    Purpose   
    =======   

    This subroutine computes the square root of the I-th eigenvalue   
    of a positive symmetric rank-one modification of a 2-by-2 diagonal   
    matrix   

               diag( D ) * diag( D ) +  RHO *  Z * transpose(Z) .   

    The diagonal entries in the array D are assumed to satisfy   

               0 <= D(i) < D(j)  for  i < j .   

    We also assume RHO > 0 and that the Euclidean norm of the vector   
    Z is one.   

    Arguments   
    =========   

    I      (input) INTEGER   
           The index of the eigenvalue to be computed.  I = 1 or I = 2.   

    D      (input) DOUBLE PRECISION array, dimension ( 2 )   
           The original eigenvalues.  We assume 0 <= D(1) < D(2).   

    Z      (input) DOUBLE PRECISION array, dimension ( 2 )   
           The components of the updating vector.   

    DELTA  (output) DOUBLE PRECISION array, dimension ( 2 )   
           Contains (D(j) - sigma_I) in its  j-th component.   
           The vector DELTA contains the information necessary   
           to construct the eigenvectors.   

    RHO    (input) DOUBLE PRECISION   
           The scalar in the symmetric updating formula.   

    DSIGMA (output) DOUBLE PRECISION   
           The computed sigma_I, the I-th updated eigenvalue.   

    WORK   (workspace) DOUBLE PRECISION array, dimension ( 2 )   
           WORK contains (D(j) + sigma_I) in its  j-th component.   

    Further Details   
    ===============   

    Based on contributions by   
       Ren-Cang Li, Computer Science Division, University of California   
       at Berkeley, USA   

    =====================================================================   


       Parameter adjustments */
    --work;
    --delta;
    --z__;
    --d__;

    /* Function Body */
    del = d__[2] - d__[1];
    delsq = del * (d__[2] + d__[1]);
    if (*i__ == 1) {
	w = *rho * 4. * (z__[2] * z__[2] / (d__[1] + d__[2] * 3.) - z__[1] * 
		z__[1] / (d__[1] * 3. + d__[2])) / del + 1.;
	if (w > 0.) {
	    b = delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	    c__ = *rho * z__[1] * z__[1] * delsq;

/*           B > ZERO, always   

             The following TAU is DSIGMA * DSIGMA - D( 1 ) * D( 1 ) */

	    tau = c__ * 2. / (b + sqrt((d__1 = b * b - c__ * 4., abs(d__1))));

/*           The following TAU is DSIGMA - D( 1 ) */

	    tau /= d__[1] + sqrt(d__[1] * d__[1] + tau);
	    *dsigma = d__[1] + tau;
	    delta[1] = -tau;
	    delta[2] = del - tau;
	    work[1] = d__[1] * 2. + tau;
	    work[2] = d__[1] + tau + d__[2];
/*           DELTA( 1 ) = -Z( 1 ) / TAU   
             DELTA( 2 ) = Z( 2 ) / ( DEL-TAU ) */
	} else {
	    b = -delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	    c__ = *rho * z__[2] * z__[2] * delsq;

/*           The following TAU is DSIGMA * DSIGMA - D( 2 ) * D( 2 ) */

	    if (b > 0.) {
		tau = c__ * -2. / (b + sqrt(b * b + c__ * 4.));
	    } else {
		tau = (b - sqrt(b * b + c__ * 4.)) / 2.;
	    }

/*           The following TAU is DSIGMA - D( 2 ) */

	    tau /= d__[2] + sqrt((d__1 = d__[2] * d__[2] + tau, abs(d__1)));
	    *dsigma = d__[2] + tau;
	    delta[1] = -(del + tau);
	    delta[2] = -tau;
	    work[1] = d__[1] + tau + d__[2];
	    work[2] = d__[2] * 2. + tau;
/*           DELTA( 1 ) = -Z( 1 ) / ( DEL+TAU )   
             DELTA( 2 ) = -Z( 2 ) / TAU */
	}
/*        TEMP = SQRT( DELTA( 1 )*DELTA( 1 )+DELTA( 2 )*DELTA( 2 ) )   
          DELTA( 1 ) = DELTA( 1 ) / TEMP   
          DELTA( 2 ) = DELTA( 2 ) / TEMP */
    } else {

/*        Now I=2 */

	b = -delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	c__ = *rho * z__[2] * z__[2] * delsq;

/*        The following TAU is DSIGMA * DSIGMA - D( 2 ) * D( 2 ) */

	if (b > 0.) {
	    tau = (b + sqrt(b * b + c__ * 4.)) / 2.;
	} else {
	    tau = c__ * 2. / (-b + sqrt(b * b + c__ * 4.));
	}

/*        The following TAU is DSIGMA - D( 2 ) */

	tau /= d__[2] + sqrt(d__[2] * d__[2] + tau);
	*dsigma = d__[2] + tau;
	delta[1] = -(del + tau);
	delta[2] = -tau;
	work[1] = d__[1] + tau + d__[2];
	work[2] = d__[2] * 2. + tau;
/*        DELTA( 1 ) = -Z( 1 ) / ( DEL+TAU )   
          DELTA( 2 ) = -Z( 2 ) / TAU   
          TEMP = SQRT( DELTA( 1 )*DELTA( 1 )+DELTA( 2 )*DELTA( 2 ) )   
          DELTA( 1 ) = DELTA( 1 ) / TEMP   
          DELTA( 2 ) = DELTA( 2 ) / TEMP */
    }
    return 0;

/*     End of DLASD5 */

} /* dlasd5_ */