/*  -- translated by f2c (version 20240504).
   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"

/* > \brief \b DLARRK computes one eigenvalue of a symmetric tridiagonal matrix T to suitable accuracy.   

    =========== DOCUMENTATION ===========   

   Online html documentation available at   
              http://www.netlib.org/lapack/explore-html/   

   > \htmlonly   
   > Download DLARRK + dependencies   
   > <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/dlarrk.
f">   
   > [TGZ]</a>   
   > <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/dlarrk.
f">   
   > [ZIP]</a>   
   > <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dlarrk.
f">   
   > [TXT]</a>   
   > \endhtmlonly   

    Definition:   
    ===========   

         SUBROUTINE DLARRK( N, IW, GL, GU,   
                             D, E2, PIVMIN, RELTOL, W, WERR, INFO)   

         INTEGER   INFO, IW, N   
         DOUBLE PRECISION    PIVMIN, RELTOL, GL, GU, W, WERR   
         DOUBLE PRECISION   D( * ), E2( * )   


   > \par Purpose:   
    =============   
   >   
   > \verbatim   
   >   
   > DLARRK computes one eigenvalue of a symmetric tridiagonal   
   > matrix T to suitable accuracy. This is an auxiliary code to be   
   > called from DSTEMR.   
   >   
   > To avoid overflow, the matrix must be scaled so that its   
   > largest element is no greater than overflow**(1/2) * underflow**(1/4) in absolute value, and for greatest
   
   > accuracy, it should not be much smaller than that.   
   >   
   > See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal   
   > Matrix", Report CS41, Computer Science Dept., Stanford   
   > University, July 21, 1966.   
   > \endverbatim   

    Arguments:   
    ==========   

   > \param[in] N   
   > \verbatim   
   >          N is INTEGER   
   >          The order of the tridiagonal matrix T.  N >= 0.   
   > \endverbatim   
   >   
   > \param[in] IW   
   > \verbatim   
   >          IW is INTEGER   
   >          The index of the eigenvalues to be returned.   
   > \endverbatim   
   >   
   > \param[in] GL   
   > \verbatim   
   >          GL is DOUBLE PRECISION   
   > \endverbatim   
   >   
   > \param[in] GU   
   > \verbatim   
   >          GU is DOUBLE PRECISION   
   >          An upper and a lower bound on the eigenvalue.   
   > \endverbatim   
   >   
   > \param[in] D   
   > \verbatim   
   >          D is DOUBLE PRECISION array, dimension (N)   
   >          The n diagonal elements of the tridiagonal matrix T.   
   > \endverbatim   
   >   
   > \param[in] E2   
   > \verbatim   
   >          E2 is DOUBLE PRECISION array, dimension (N-1)   
   >          The (n-1) squared off-diagonal elements of the tridiagonal matrix T.   
   > \endverbatim   
   >   
   > \param[in] PIVMIN   
   > \verbatim   
   >          PIVMIN is DOUBLE PRECISION   
   >          The minimum pivot allowed in the Sturm sequence for T.   
   > \endverbatim   
   >   
   > \param[in] RELTOL   
   > \verbatim   
   >          RELTOL is DOUBLE PRECISION   
   >          The minimum relative width of an interval.  When an interval   
   >          is narrower than RELTOL times the larger (in   
   >          magnitude) endpoint, then it is considered to be   
   >          sufficiently small, i.e., converged.  Note: this should   
   >          always be at least radix*machine epsilon.   
   > \endverbatim   
   >   
   > \param[out] W   
   > \verbatim   
   >          W is DOUBLE PRECISION   
   > \endverbatim   
   >   
   > \param[out] WERR   
   > \verbatim   
   >          WERR is DOUBLE PRECISION   
   >          The error bound on the corresponding eigenvalue approximation   
   >          in W.   
   > \endverbatim   
   >   
   > \param[out] INFO   
   > \verbatim   
   >          INFO is INTEGER   
   >          = 0:       Eigenvalue converged   
   >          = -1:      Eigenvalue did NOT converge   
   > \endverbatim   

   > \par Internal Parameters:   
    =========================   
   >   
   > \verbatim   
   >  FUDGE   DOUBLE PRECISION, default = 2   
   >          A "fudge factor" to widen the Gershgorin intervals.   
   > \endverbatim   

    Authors:   
    ========   

   > \author Univ. of Tennessee   
   > \author Univ. of California Berkeley   
   > \author Univ. of Colorado Denver   
   > \author NAG Ltd.   

   > \date September 2012   

   > \ingroup auxOTHERauxiliary   

    =====================================================================   
   Subroutine */ int igraphdlarrk_(integer *n, integer *iw, doublereal *gl, 
	doublereal *gu, doublereal *d__, doublereal *e2, doublereal *pivmin, 
	doublereal *reltol, doublereal *w, doublereal *werr, integer *info)
{
    /* System generated locals */
    integer i__1;
    doublereal d__1, d__2;

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

    /* Local variables */
    integer i__, it;
    doublereal mid, eps, tmp1, tmp2, left, atoli, right;
    integer itmax;
    doublereal rtoli, tnorm;
    extern doublereal igraphdlamch_(char *);
    integer negcnt;


/*  -- LAPACK auxiliary routine (version 3.4.2) --   
    -- LAPACK is a software package provided by Univ. of Tennessee,    --   
    -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--   
       September 2012   


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


       Get machine constants   
       Parameter adjustments */
    --e2;
    --d__;

    /* Function Body */
    eps = igraphdlamch_("P");
/* Computing MAX */
    d__1 = abs(*gl), d__2 = abs(*gu);
    tnorm = max(d__1,d__2);
    rtoli = *reltol;
    atoli = *pivmin * 4.;
    itmax = (integer) ((log(tnorm + *pivmin) - log(*pivmin)) / log(2.)) + 2;
    *info = -1;
    left = *gl - tnorm * 2. * eps * *n - *pivmin * 4.;
    right = *gu + tnorm * 2. * eps * *n + *pivmin * 4.;
    it = 0;
L10:

/*     Check if interval converged or maximum number of iterations reached */

    tmp1 = (d__1 = right - left, abs(d__1));
/* Computing MAX */
    d__1 = abs(right), d__2 = abs(left);
    tmp2 = max(d__1,d__2);
/* Computing MAX */
    d__1 = max(atoli,*pivmin), d__2 = rtoli * tmp2;
    if (tmp1 < max(d__1,d__2)) {
	*info = 0;
	goto L30;
    }
    if (it > itmax) {
	goto L30;
    }

/*     Count number of negative pivots for mid-point */

    ++it;
    mid = (left + right) * .5;
    negcnt = 0;
    tmp1 = d__[1] - mid;
    if (abs(tmp1) < *pivmin) {
	tmp1 = -(*pivmin);
    }
    if (tmp1 <= 0.) {
	++negcnt;
    }

    i__1 = *n;
    for (i__ = 2; i__ <= i__1; ++i__) {
	tmp1 = d__[i__] - e2[i__ - 1] / tmp1 - mid;
	if (abs(tmp1) < *pivmin) {
	    tmp1 = -(*pivmin);
	}
	if (tmp1 <= 0.) {
	    ++negcnt;
	}
/* L20: */
    }
    if (negcnt >= *iw) {
	right = mid;
    } else {
	left = mid;
    }
    goto L10;
L30:

/*     Converged or maximum number of iterations reached */

    *w = (left + right) * .5;
    *werr = (d__1 = right - left, abs(d__1)) * .5;
    return 0;

/*     End of DLARRK */

} /* igraphdlarrk_ */

