dstect.c

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/* dstect.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 dstect_(integer *n, doublereal *a, doublereal *b, 
        doublereal *shift, integer *num)
{
    /* System generated locals */
    integer i__1;
    doublereal d__1, d__2, d__3, d__4;

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

    /* Local variables */
    integer i__;
    doublereal u, m1, m2, mx, tmp, tom, sun, sov, unfl, ovfl, ssun;
    extern doublereal dlamch_(char *);
    doublereal sshift;


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

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

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

/*     DSTECT counts the number NUM of eigenvalues of a tridiagonal */
/*     matrix T which are less than or equal to SHIFT. T has */
/*     diagonal entries A(1), ... , A(N), and offdiagonal entries */
/*     B(1), ..., B(N-1). */
/*     See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal */
/*     Matrix", Report CS41, Computer Science Dept., Stanford */
/*     University, July 21, 1966 */

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

/*  N       (input) INTEGER */
/*          The dimension of the tridiagonal matrix T. */

/*  A       (input) DOUBLE PRECISION array, dimension (N) */
/*          The diagonal entries of the tridiagonal matrix T. */

/*  B       (input) DOUBLE PRECISION array, dimension (N-1) */
/*          The offdiagonal entries of the tridiagonal matrix T. */

/*  SHIFT   (input) DOUBLE PRECISION */
/*          The shift, used as described under Purpose. */

/*  NUM     (output) INTEGER */
/*          The number of eigenvalues of T less than or equal */
/*          to SHIFT. */

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

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

/*     Get machine constants */

    /* Parameter adjustments */
    --b;
    --a;

    /* Function Body */
    unfl = dlamch_("Safe minimum");
    ovfl = dlamch_("Overflow");

/*     Find largest entry */

    mx = abs(a[1]);
    i__1 = *n - 1;
    for (i__ = 1; i__ <= i__1; ++i__) {
/* Computing MAX */
        d__3 = mx, d__4 = (d__1 = a[i__ + 1], abs(d__1)), d__3 = max(d__3,
                d__4), d__4 = (d__2 = b[i__], abs(d__2));
        mx = max(d__3,d__4);
/* L10: */
    }

/*     Handle easy cases, including zero matrix */

    if (*shift >= mx * 3.) {
        *num = *n;
        return 0;
    }
    if (*shift < mx * -3.) {
        *num = 0;
        return 0;
    }

/*     Compute scale factors as in Kahan's report */
/*     At this point, MX .NE. 0 so we can divide by it */

    sun = sqrt(unfl);
    ssun = sqrt(sun);
    sov = sqrt(ovfl);
    tom = ssun * sov;
    if (mx <= 1.) {
        m1 = 1. / mx;
        m2 = tom;
    } else {
        m1 = 1.;
        m2 = tom / mx;
    }

/*     Begin counting */

    *num = 0;
    sshift = *shift * m1 * m2;
    u = a[1] * m1 * m2 - sshift;
    if (u <= sun) {
        if (u <= 0.) {
            ++(*num);
            if (u > -sun) {
                u = -sun;
            }
        } else {
            u = sun;
        }
    }
    i__1 = *n;
    for (i__ = 2; i__ <= i__1; ++i__) {
        tmp = b[i__ - 1] * m1 * m2;
        u = a[i__] * m1 * m2 - tmp * (tmp / u) - sshift;
        if (u <= sun) {
            if (u <= 0.) {
                ++(*num);
                if (u > -sun) {
                    u = -sun;
                }
            } else {
                u = sun;
            }
        }
/* L20: */
    }
    return 0;

/*     End of DSTECT */

} /* dstect_ */