function b = zerotx(F, ab, varargin)
a = ab(1);
b = ab(2);
fa = F(a, varargin{:});
fb = F(b, varargin{:});
if (sign(fa) == sign(fb))
    b = -1e10;
    return;
end
c = a;
fc = fa;
d = b - c;
e = d;

while fb ~= 0
    if (sign(fa) == sign(fb))
        a = c; fa = fc;
        d = b - c; e = d;
    end
    if abs(fa) < abs(fb)
        c = b; b = a; a = c;
        fc = fb; fb = fa; fa = fc;
    end
    m = 0.5 * (a - b);
    tol = 2.0 * eps * max(abs(b), 1.0);
    if (abs(m) <= tol) | (fb == 0.0)
        break;
    end
    if (abs(m) <= tol) | (abs(fc) <= abs(fb))
        d = m; e = m;
    else
        s = fb / fc;
        if (a == c)
            p = 2.0 * m * s;
            q = 1.0 - s;
        else
            q = fc / fa;
            r = fb / fa;
            p = s * (2.0 * m * q * (q - r) - (b - c) * (r - 1.0));
            q = (q - 1.0) * (r - 1.0) * (s - 1.0);
        end
        if p > 0, q = -q; else p = -p; end
        if (2.0 * p < 3.0 * m * q - abs(tol * q)) & (p < abs(0.5 * e * q))
            e = d;
            d = p / q;
        else
            d = m;
            e = m;
        end
    end
    
    c = b;
    fc = fb;
    if abs(d) > tol
        b = b + d;
    else
        b = b - sign(b - a) * tol;
    end
    fb = F(b, varargin{:});
end