dlaed4.c

Go to the documentation of this file.

/* dlaed4.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"
#include "blaswrap.h"

/* Subroutine */ int dlaed4_(integer *n, integer *i__, doublereal *d__, 
        doublereal *z__, doublereal *delta, doublereal *rho, doublereal *dlam, 
         integer *info)
{
    /* System generated locals */
    integer i__1;
    doublereal d__1;

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

    /* Local variables */
    doublereal a, b, c__;
    integer j;
    doublereal w;
    integer ii;
    doublereal dw, zz[3];
    integer ip1;
    doublereal del, eta, phi, eps, tau, psi;
    integer iim1, iip1;
    doublereal dphi, dpsi;
    integer iter;
    doublereal temp, prew, temp1, dltlb, dltub, midpt;
    integer niter;
    logical swtch;
    extern /* Subroutine */ int dlaed5_(integer *, doublereal *, doublereal *, 
             doublereal *, doublereal *, doublereal *), dlaed6_(integer *, 
            logical *, doublereal *, doublereal *, doublereal *, doublereal *, 
             doublereal *, integer *);
    logical swtch3;
    extern doublereal dlamch_(char *);
    logical orgati;
    doublereal erretm, rhoinv;


/*  -- LAPACK routine (version 3.2) -- */
/*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/*     November 2006 */

/*     .. Scalar Arguments .. */
/*     .. */
/*     .. Array Arguments .. */
/*     .. */

/*  Purpose */
/*  ======= */

/*  This subroutine computes the I-th updated eigenvalue of a symmetric */
/*  rank-one modification to a diagonal matrix whose elements are */
/*  given in the array d, and that */

/*             D(i) < D(j)  for  i < j */

/*  and that RHO > 0.  This is arranged by the calling routine, and is */
/*  no loss in generality.  The rank-one modified system is thus */

/*             diag( D )  +  RHO *  Z * Z_transpose. */

/*  where we assume the Euclidean norm of Z is 1. */

/*  The method consists of approximating the rational functions in the */
/*  secular equation by simpler interpolating rational functions. */

/*  Arguments */
/*  ========= */

/*  N      (input) INTEGER */
/*         The length of all arrays. */

/*  I      (input) INTEGER */
/*         The index of the eigenvalue to be computed.  1 <= I <= N. */

/*  D      (input) DOUBLE PRECISION array, dimension (N) */
/*         The original eigenvalues.  It is assumed that they are in */
/*         order, D(I) < D(J)  for I < J. */

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

/*  DELTA  (output) DOUBLE PRECISION array, dimension (N) */
/*         If N .GT. 2, DELTA contains (D(j) - lambda_I) in its  j-th */
/*         component.  If N = 1, then DELTA(1) = 1. If N = 2, see DLAED5 */
/*         for detail. The vector DELTA contains the information necessary */
/*         to construct the eigenvectors by DLAED3 and DLAED9. */

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

/*  DLAM   (output) DOUBLE PRECISION */
/*         The computed lambda_I, the I-th updated eigenvalue. */

/*  INFO   (output) INTEGER */
/*         = 0:  successful exit */
/*         > 0:  if INFO = 1, the updating process failed. */

/*  Internal Parameters */
/*  =================== */

/*  Logical variable ORGATI (origin-at-i?) is used for distinguishing */
/*  whether D(i) or D(i+1) is treated as the origin. */

/*            ORGATI = .true.    origin at i */
/*            ORGATI = .false.   origin at i+1 */

/*   Logical variable SWTCH3 (switch-for-3-poles?) is for noting */
/*   if we are working with THREE poles! */

/*   MAXIT is the maximum number of iterations allowed for each */
/*   eigenvalue. */

/*  Further Details */
/*  =============== */

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

/*  ===================================================================== */

/*     .. Parameters .. */
/*     .. */
/*     .. Local Scalars .. */
/*     .. */
/*     .. Local Arrays .. */
/*     .. */
/*     .. External Functions .. */
/*     .. */
/*     .. External Subroutines .. */
/*     .. */
/*     .. Intrinsic Functions .. */
/*     .. */
/*     .. Executable Statements .. */

/*     Since this routine is called in an inner loop, we do no argument */
/*     checking. */

/*     Quick return for N=1 and 2. */

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

    /* Function Body */
    *info = 0;
    if (*n == 1) {

/*         Presumably, I=1 upon entry */

        *dlam = d__[1] + *rho * z__[1] * z__[1];
        delta[1] = 1.;
        return 0;
    }
    if (*n == 2) {
        dlaed5_(i__, &d__[1], &z__[1], &delta[1], rho, dlam);
        return 0;
    }

/*     Compute machine epsilon */

    eps = dlamch_("Epsilon");
    rhoinv = 1. / *rho;

/*     The case I = N */

    if (*i__ == *n) {

/*        Initialize some basic variables */

        ii = *n - 1;
        niter = 1;

/*        Calculate initial guess */

        midpt = *rho / 2.;

/*        If ||Z||_2 is not one, then TEMP should be set to */
/*        RHO * ||Z||_2^2 / TWO */

        i__1 = *n;
        for (j = 1; j <= i__1; ++j) {
            delta[j] = d__[j] - d__[*i__] - midpt;
/* L10: */
        }

        psi = 0.;
        i__1 = *n - 2;
        for (j = 1; j <= i__1; ++j) {
            psi += z__[j] * z__[j] / delta[j];
/* L20: */
        }

        c__ = rhoinv + psi;
        w = c__ + z__[ii] * z__[ii] / delta[ii] + z__[*n] * z__[*n] / delta[*
                n];

        if (w <= 0.) {
            temp = z__[*n - 1] * z__[*n - 1] / (d__[*n] - d__[*n - 1] + *rho) 
                    + z__[*n] * z__[*n] / *rho;
            if (c__ <= temp) {
                tau = *rho;
            } else {
                del = d__[*n] - d__[*n - 1];
                a = -c__ * del + z__[*n - 1] * z__[*n - 1] + z__[*n] * z__[*n]
                        ;
                b = z__[*n] * z__[*n] * del;
                if (a < 0.) {
                    tau = b * 2. / (sqrt(a * a + b * 4. * c__) - a);
                } else {
                    tau = (a + sqrt(a * a + b * 4. * c__)) / (c__ * 2.);
                }
            }

/*           It can be proved that */
/*               D(N)+RHO/2 <= LAMBDA(N) < D(N)+TAU <= D(N)+RHO */

            dltlb = midpt;
            dltub = *rho;
        } else {
            del = d__[*n] - d__[*n - 1];
            a = -c__ * del + z__[*n - 1] * z__[*n - 1] + z__[*n] * z__[*n];
            b = z__[*n] * z__[*n] * del;
            if (a < 0.) {
                tau = b * 2. / (sqrt(a * a + b * 4. * c__) - a);
            } else {
                tau = (a + sqrt(a * a + b * 4. * c__)) / (c__ * 2.);
            }

/*           It can be proved that */
/*               D(N) < D(N)+TAU < LAMBDA(N) < D(N)+RHO/2 */

            dltlb = 0.;
            dltub = midpt;
        }

        i__1 = *n;
        for (j = 1; j <= i__1; ++j) {
            delta[j] = d__[j] - d__[*i__] - tau;
/* L30: */
        }

/*        Evaluate PSI and the derivative DPSI */

        dpsi = 0.;
        psi = 0.;
        erretm = 0.;
        i__1 = ii;
        for (j = 1; j <= i__1; ++j) {
            temp = z__[j] / delta[j];
            psi += z__[j] * temp;
            dpsi += temp * temp;
            erretm += psi;
/* L40: */
        }
        erretm = abs(erretm);

/*        Evaluate PHI and the derivative DPHI */

        temp = z__[*n] / delta[*n];
        phi = z__[*n] * temp;
        dphi = temp * temp;
        erretm = (-phi - psi) * 8. + erretm - phi + rhoinv + abs(tau) * (dpsi 
                + dphi);

        w = rhoinv + phi + psi;

/*        Test for convergence */

        if (abs(w) <= eps * erretm) {
            *dlam = d__[*i__] + tau;
            goto L250;
        }

        if (w <= 0.) {
            dltlb = max(dltlb,tau);
        } else {
            dltub = min(dltub,tau);
        }

/*        Calculate the new step */

        ++niter;
        c__ = w - delta[*n - 1] * dpsi - delta[*n] * dphi;
        a = (delta[*n - 1] + delta[*n]) * w - delta[*n - 1] * delta[*n] * (
                dpsi + dphi);
        b = delta[*n - 1] * delta[*n] * w;
        if (c__ < 0.) {
            c__ = abs(c__);
        }
        if (c__ == 0.) {
/*          ETA = B/A */
/*           ETA = RHO - TAU */
            eta = dltub - tau;
        } else if (a >= 0.) {
            eta = (a + sqrt((d__1 = a * a - b * 4. * c__, abs(d__1)))) / (c__ 
                    * 2.);
        } else {
            eta = b * 2. / (a - sqrt((d__1 = a * a - b * 4. * c__, abs(d__1)))
                    );
        }

/*        Note, eta should be positive if w is negative, and */
/*        eta should be negative otherwise. However, */
/*        if for some reason caused by roundoff, eta*w > 0, */
/*        we simply use one Newton step instead. This way */
/*        will guarantee eta*w < 0. */

        if (w * eta > 0.) {
            eta = -w / (dpsi + dphi);
        }
        temp = tau + eta;
        if (temp > dltub || temp < dltlb) {
            if (w < 0.) {
                eta = (dltub - tau) / 2.;
            } else {
                eta = (dltlb - tau) / 2.;
            }
        }
        i__1 = *n;
        for (j = 1; j <= i__1; ++j) {
            delta[j] -= eta;
/* L50: */
        }

        tau += eta;

/*        Evaluate PSI and the derivative DPSI */

        dpsi = 0.;
        psi = 0.;
        erretm = 0.;
        i__1 = ii;
        for (j = 1; j <= i__1; ++j) {
            temp = z__[j] / delta[j];
            psi += z__[j] * temp;
            dpsi += temp * temp;
            erretm += psi;
/* L60: */
        }
        erretm = abs(erretm);

/*        Evaluate PHI and the derivative DPHI */

        temp = z__[*n] / delta[*n];
        phi = z__[*n] * temp;
        dphi = temp * temp;
        erretm = (-phi - psi) * 8. + erretm - phi + rhoinv + abs(tau) * (dpsi 
                + dphi);

        w = rhoinv + phi + psi;

/*        Main loop to update the values of the array   DELTA */

        iter = niter + 1;

        for (niter = iter; niter <= 30; ++niter) {

/*           Test for convergence */

            if (abs(w) <= eps * erretm) {
                *dlam = d__[*i__] + tau;
                goto L250;
            }

            if (w <= 0.) {
                dltlb = max(dltlb,tau);
            } else {
                dltub = min(dltub,tau);
            }

/*           Calculate the new step */

            c__ = w - delta[*n - 1] * dpsi - delta[*n] * dphi;
            a = (delta[*n - 1] + delta[*n]) * w - delta[*n - 1] * delta[*n] * 
                    (dpsi + dphi);
            b = delta[*n - 1] * delta[*n] * w;
            if (a >= 0.) {
                eta = (a + sqrt((d__1 = a * a - b * 4. * c__, abs(d__1)))) / (
                        c__ * 2.);
            } else {
                eta = b * 2. / (a - sqrt((d__1 = a * a - b * 4. * c__, abs(
                        d__1))));
            }

/*           Note, eta should be positive if w is negative, and */
/*           eta should be negative otherwise. However, */
/*           if for some reason caused by roundoff, eta*w > 0, */
/*           we simply use one Newton step instead. This way */
/*           will guarantee eta*w < 0. */

            if (w * eta > 0.) {
                eta = -w / (dpsi + dphi);
            }
            temp = tau + eta;
            if (temp > dltub || temp < dltlb) {
                if (w < 0.) {
                    eta = (dltub - tau) / 2.;
                } else {
                    eta = (dltlb - tau) / 2.;
                }
            }
            i__1 = *n;
            for (j = 1; j <= i__1; ++j) {
                delta[j] -= eta;
/* L70: */
            }

            tau += eta;

/*           Evaluate PSI and the derivative DPSI */

            dpsi = 0.;
            psi = 0.;
            erretm = 0.;
            i__1 = ii;
            for (j = 1; j <= i__1; ++j) {
                temp = z__[j] / delta[j];
                psi += z__[j] * temp;
                dpsi += temp * temp;
                erretm += psi;
/* L80: */
            }
            erretm = abs(erretm);

/*           Evaluate PHI and the derivative DPHI */

            temp = z__[*n] / delta[*n];
            phi = z__[*n] * temp;
            dphi = temp * temp;
            erretm = (-phi - psi) * 8. + erretm - phi + rhoinv + abs(tau) * (
                    dpsi + dphi);

            w = rhoinv + phi + psi;
/* L90: */
        }

/*        Return with INFO = 1, NITER = MAXIT and not converged */

        *info = 1;
        *dlam = d__[*i__] + tau;
        goto L250;

/*        End for the case I = N */

    } else {

/*        The case for I < N */

        niter = 1;
        ip1 = *i__ + 1;

/*        Calculate initial guess */

        del = d__[ip1] - d__[*i__];
        midpt = del / 2.;
        i__1 = *n;
        for (j = 1; j <= i__1; ++j) {
            delta[j] = d__[j] - d__[*i__] - midpt;
/* L100: */
        }

        psi = 0.;
        i__1 = *i__ - 1;
        for (j = 1; j <= i__1; ++j) {
            psi += z__[j] * z__[j] / delta[j];
/* L110: */
        }

        phi = 0.;
        i__1 = *i__ + 2;
        for (j = *n; j >= i__1; --j) {
            phi += z__[j] * z__[j] / delta[j];
/* L120: */
        }
        c__ = rhoinv + psi + phi;
        w = c__ + z__[*i__] * z__[*i__] / delta[*i__] + z__[ip1] * z__[ip1] / 
                delta[ip1];

        if (w > 0.) {

/*           d(i)< the ith eigenvalue < (d(i)+d(i+1))/2 */

/*           We choose d(i) as origin. */

            orgati = TRUE_;
            a = c__ * del + z__[*i__] * z__[*i__] + z__[ip1] * z__[ip1];
            b = z__[*i__] * z__[*i__] * del;
            if (a > 0.) {
                tau = b * 2. / (a + sqrt((d__1 = a * a - b * 4. * c__, abs(
                        d__1))));
            } else {
                tau = (a - sqrt((d__1 = a * a - b * 4. * c__, abs(d__1)))) / (
                        c__ * 2.);
            }
            dltlb = 0.;
            dltub = midpt;
        } else {

/*           (d(i)+d(i+1))/2 <= the ith eigenvalue < d(i+1) */

/*           We choose d(i+1) as origin. */

            orgati = FALSE_;
            a = c__ * del - z__[*i__] * z__[*i__] - z__[ip1] * z__[ip1];
            b = z__[ip1] * z__[ip1] * del;
            if (a < 0.) {
                tau = b * 2. / (a - sqrt((d__1 = a * a + b * 4. * c__, abs(
                        d__1))));
            } else {
                tau = -(a + sqrt((d__1 = a * a + b * 4. * c__, abs(d__1)))) / 
                        (c__ * 2.);
            }
            dltlb = -midpt;
            dltub = 0.;
        }

        if (orgati) {
            i__1 = *n;
            for (j = 1; j <= i__1; ++j) {
                delta[j] = d__[j] - d__[*i__] - tau;
/* L130: */
            }
        } else {
            i__1 = *n;
            for (j = 1; j <= i__1; ++j) {
                delta[j] = d__[j] - d__[ip1] - tau;
/* L140: */
            }
        }
        if (orgati) {
            ii = *i__;
        } else {
            ii = *i__ + 1;
        }
        iim1 = ii - 1;
        iip1 = ii + 1;

/*        Evaluate PSI and the derivative DPSI */

        dpsi = 0.;
        psi = 0.;
        erretm = 0.;
        i__1 = iim1;
        for (j = 1; j <= i__1; ++j) {
            temp = z__[j] / delta[j];
            psi += z__[j] * temp;
            dpsi += temp * temp;
            erretm += psi;
/* L150: */
        }
        erretm = abs(erretm);

/*        Evaluate PHI and the derivative DPHI */

        dphi = 0.;
        phi = 0.;
        i__1 = iip1;
        for (j = *n; j >= i__1; --j) {
            temp = z__[j] / delta[j];
            phi += z__[j] * temp;
            dphi += temp * temp;
            erretm += phi;
/* L160: */
        }

        w = rhoinv + phi + psi;

/*        W is the value of the secular function with */
/*        its ii-th element removed. */

        swtch3 = FALSE_;
        if (orgati) {
            if (w < 0.) {
                swtch3 = TRUE_;
            }
        } else {
            if (w > 0.) {
                swtch3 = TRUE_;
            }
        }
        if (ii == 1 || ii == *n) {
            swtch3 = FALSE_;
        }

        temp = z__[ii] / delta[ii];
        dw = dpsi + dphi + temp * temp;
        temp = z__[ii] * temp;
        w += temp;
        erretm = (phi - psi) * 8. + erretm + rhoinv * 2. + abs(temp) * 3. + 
                abs(tau) * dw;

/*        Test for convergence */

        if (abs(w) <= eps * erretm) {
            if (orgati) {
                *dlam = d__[*i__] + tau;
            } else {
                *dlam = d__[ip1] + tau;
            }
            goto L250;
        }

        if (w <= 0.) {
            dltlb = max(dltlb,tau);
        } else {
            dltub = min(dltub,tau);
        }

/*        Calculate the new step */

        ++niter;
        if (! swtch3) {
            if (orgati) {
/* Computing 2nd power */
                d__1 = z__[*i__] / delta[*i__];
                c__ = w - delta[ip1] * dw - (d__[*i__] - d__[ip1]) * (d__1 * 
                        d__1);
            } else {
/* Computing 2nd power */
                d__1 = z__[ip1] / delta[ip1];
                c__ = w - delta[*i__] * dw - (d__[ip1] - d__[*i__]) * (d__1 * 
                        d__1);
            }
            a = (delta[*i__] + delta[ip1]) * w - delta[*i__] * delta[ip1] * 
                    dw;
            b = delta[*i__] * delta[ip1] * w;
            if (c__ == 0.) {
                if (a == 0.) {
                    if (orgati) {
                        a = z__[*i__] * z__[*i__] + delta[ip1] * delta[ip1] * 
                                (dpsi + dphi);
                    } else {
                        a = z__[ip1] * z__[ip1] + delta[*i__] * delta[*i__] * 
                                (dpsi + dphi);
                    }
                }
                eta = b / a;
            } else if (a <= 0.) {
                eta = (a - sqrt((d__1 = a * a - b * 4. * c__, abs(d__1)))) / (
                        c__ * 2.);
            } else {
                eta = b * 2. / (a + sqrt((d__1 = a * a - b * 4. * c__, abs(
                        d__1))));
            }
        } else {

/*           Interpolation using THREE most relevant poles */

            temp = rhoinv + psi + phi;
            if (orgati) {
                temp1 = z__[iim1] / delta[iim1];
                temp1 *= temp1;
                c__ = temp - delta[iip1] * (dpsi + dphi) - (d__[iim1] - d__[
                        iip1]) * temp1;
                zz[0] = z__[iim1] * z__[iim1];
                zz[2] = delta[iip1] * delta[iip1] * (dpsi - temp1 + dphi);
            } else {
                temp1 = z__[iip1] / delta[iip1];
                temp1 *= temp1;
                c__ = temp - delta[iim1] * (dpsi + dphi) - (d__[iip1] - d__[
                        iim1]) * temp1;
                zz[0] = delta[iim1] * delta[iim1] * (dpsi + (dphi - temp1));
                zz[2] = z__[iip1] * z__[iip1];
            }
            zz[1] = z__[ii] * z__[ii];
            dlaed6_(&niter, &orgati, &c__, &delta[iim1], zz, &w, &eta, info);
            if (*info != 0) {
                goto L250;
            }
        }

/*        Note, eta should be positive if w is negative, and */
/*        eta should be negative otherwise. However, */
/*        if for some reason caused by roundoff, eta*w > 0, */
/*        we simply use one Newton step instead. This way */
/*        will guarantee eta*w < 0. */

        if (w * eta >= 0.) {
            eta = -w / dw;
        }
        temp = tau + eta;
        if (temp > dltub || temp < dltlb) {
            if (w < 0.) {
                eta = (dltub - tau) / 2.;
            } else {
                eta = (dltlb - tau) / 2.;
            }
        }

        prew = w;

        i__1 = *n;
        for (j = 1; j <= i__1; ++j) {
            delta[j] -= eta;
/* L180: */
        }

/*        Evaluate PSI and the derivative DPSI */

        dpsi = 0.;
        psi = 0.;
        erretm = 0.;
        i__1 = iim1;
        for (j = 1; j <= i__1; ++j) {
            temp = z__[j] / delta[j];
            psi += z__[j] * temp;
            dpsi += temp * temp;
            erretm += psi;
/* L190: */
        }
        erretm = abs(erretm);

/*        Evaluate PHI and the derivative DPHI */

        dphi = 0.;
        phi = 0.;
        i__1 = iip1;
        for (j = *n; j >= i__1; --j) {
            temp = z__[j] / delta[j];
            phi += z__[j] * temp;
            dphi += temp * temp;
            erretm += phi;
/* L200: */
        }

        temp = z__[ii] / delta[ii];
        dw = dpsi + dphi + temp * temp;
        temp = z__[ii] * temp;
        w = rhoinv + phi + psi + temp;
        erretm = (phi - psi) * 8. + erretm + rhoinv * 2. + abs(temp) * 3. + (
                d__1 = tau + eta, abs(d__1)) * dw;

        swtch = FALSE_;
        if (orgati) {
            if (-w > abs(prew) / 10.) {
                swtch = TRUE_;
            }
        } else {
            if (w > abs(prew) / 10.) {
                swtch = TRUE_;
            }
        }

        tau += eta;

/*        Main loop to update the values of the array   DELTA */

        iter = niter + 1;

        for (niter = iter; niter <= 30; ++niter) {

/*           Test for convergence */

            if (abs(w) <= eps * erretm) {
                if (orgati) {
                    *dlam = d__[*i__] + tau;
                } else {
                    *dlam = d__[ip1] + tau;
                }
                goto L250;
            }

            if (w <= 0.) {
                dltlb = max(dltlb,tau);
            } else {
                dltub = min(dltub,tau);
            }

/*           Calculate the new step */

            if (! swtch3) {
                if (! swtch) {
                    if (orgati) {
/* Computing 2nd power */
                        d__1 = z__[*i__] / delta[*i__];
                        c__ = w - delta[ip1] * dw - (d__[*i__] - d__[ip1]) * (
                                d__1 * d__1);
                    } else {
/* Computing 2nd power */
                        d__1 = z__[ip1] / delta[ip1];
                        c__ = w - delta[*i__] * dw - (d__[ip1] - d__[*i__]) * 
                                (d__1 * d__1);
                    }
                } else {
                    temp = z__[ii] / delta[ii];
                    if (orgati) {
                        dpsi += temp * temp;
                    } else {
                        dphi += temp * temp;
                    }
                    c__ = w - delta[*i__] * dpsi - delta[ip1] * dphi;
                }
                a = (delta[*i__] + delta[ip1]) * w - delta[*i__] * delta[ip1] 
                        * dw;
                b = delta[*i__] * delta[ip1] * w;
                if (c__ == 0.) {
                    if (a == 0.) {
                        if (! swtch) {
                            if (orgati) {
                                a = z__[*i__] * z__[*i__] + delta[ip1] * 
                                        delta[ip1] * (dpsi + dphi);
                            } else {
                                a = z__[ip1] * z__[ip1] + delta[*i__] * delta[
                                        *i__] * (dpsi + dphi);
                            }
                        } else {
                            a = delta[*i__] * delta[*i__] * dpsi + delta[ip1] 
                                    * delta[ip1] * dphi;
                        }
                    }
                    eta = b / a;
                } else if (a <= 0.) {
                    eta = (a - sqrt((d__1 = a * a - b * 4. * c__, abs(d__1))))
                             / (c__ * 2.);
                } else {
                    eta = b * 2. / (a + sqrt((d__1 = a * a - b * 4. * c__, 
                            abs(d__1))));
                }
            } else {

/*              Interpolation using THREE most relevant poles */

                temp = rhoinv + psi + phi;
                if (swtch) {
                    c__ = temp - delta[iim1] * dpsi - delta[iip1] * dphi;
                    zz[0] = delta[iim1] * delta[iim1] * dpsi;
                    zz[2] = delta[iip1] * delta[iip1] * dphi;
                } else {
                    if (orgati) {
                        temp1 = z__[iim1] / delta[iim1];
                        temp1 *= temp1;
                        c__ = temp - delta[iip1] * (dpsi + dphi) - (d__[iim1] 
                                - d__[iip1]) * temp1;
                        zz[0] = z__[iim1] * z__[iim1];
                        zz[2] = delta[iip1] * delta[iip1] * (dpsi - temp1 + 
                                dphi);
                    } else {
                        temp1 = z__[iip1] / delta[iip1];
                        temp1 *= temp1;
                        c__ = temp - delta[iim1] * (dpsi + dphi) - (d__[iip1] 
                                - d__[iim1]) * temp1;
                        zz[0] = delta[iim1] * delta[iim1] * (dpsi + (dphi - 
                                temp1));
                        zz[2] = z__[iip1] * z__[iip1];
                    }
                }
                dlaed6_(&niter, &orgati, &c__, &delta[iim1], zz, &w, &eta, 
                        info);
                if (*info != 0) {
                    goto L250;
                }
            }

/*           Note, eta should be positive if w is negative, and */
/*           eta should be negative otherwise. However, */
/*           if for some reason caused by roundoff, eta*w > 0, */
/*           we simply use one Newton step instead. This way */
/*           will guarantee eta*w < 0. */

            if (w * eta >= 0.) {
                eta = -w / dw;
            }
            temp = tau + eta;
            if (temp > dltub || temp < dltlb) {
                if (w < 0.) {
                    eta = (dltub - tau) / 2.;
                } else {
                    eta = (dltlb - tau) / 2.;
                }
            }

            i__1 = *n;
            for (j = 1; j <= i__1; ++j) {
                delta[j] -= eta;
/* L210: */
            }

            tau += eta;
            prew = w;

/*           Evaluate PSI and the derivative DPSI */

            dpsi = 0.;
            psi = 0.;
            erretm = 0.;
            i__1 = iim1;
            for (j = 1; j <= i__1; ++j) {
                temp = z__[j] / delta[j];
                psi += z__[j] * temp;
                dpsi += temp * temp;
                erretm += psi;
/* L220: */
            }
            erretm = abs(erretm);

/*           Evaluate PHI and the derivative DPHI */

            dphi = 0.;
            phi = 0.;
            i__1 = iip1;
            for (j = *n; j >= i__1; --j) {
                temp = z__[j] / delta[j];
                phi += z__[j] * temp;
                dphi += temp * temp;
                erretm += phi;
/* L230: */
            }

            temp = z__[ii] / delta[ii];
            dw = dpsi + dphi + temp * temp;
            temp = z__[ii] * temp;
            w = rhoinv + phi + psi + temp;
            erretm = (phi - psi) * 8. + erretm + rhoinv * 2. + abs(temp) * 3. 
                    + abs(tau) * dw;
            if (w * prew > 0. && abs(w) > abs(prew) / 10.) {
                swtch = ! swtch;
            }

/* L240: */
        }

/*        Return with INFO = 1, NITER = MAXIT and not converged */

        *info = 1;
        if (orgati) {
            *dlam = d__[*i__] + tau;
        } else {
            *dlam = d__[ip1] + tau;
        }

    }

L250:

    return 0;

/*     End of DLAED4 */

} /* dlaed4_ */