double zbrent(double x1, double x2, double tol)
{
	double a,b,fa,fb,fc,c,d,e,tol1,xm,q,r,p,eps,s,aaa,tt;
	int t,itmax,iter;
    itmax = 100;
    eps = 0.00000003;
    a = x1;
    b = x2;
    fa = func(a);
    fb = func(b);
    if ((fb * fa) > 0.0)
	{
        cout<<"root must be bracketed for zbrent"<<endl;
    }
    fc = fb;
    for (iter = 1; iter<=itmax; iter++)
	{
        if ((fb * fc) > 0.0)
		{
            c = a;
            fc = fa;
            d = b - a;
            e = d;
        }
        if (fabs(fc) < fabs(fb))
		{
            a = b;
            b = c;
            c = a;
            fa = fb;
            fb = fc;
            fc = fa;
        }
        tol1 = 2.0 * eps * fabs(b) + 0.5 * tol;
        xm = 0.5 * (c - b);
        if ((fabs(xm) <= tol1) || (fb == 0.0))
		{
			tt=b;
            return tt;
			exit(1);
            
        }
        if ((fabs(e) >= tol1) && (fabs(fa) > fabs(fb)))
		{
            s = fb / fa;
            if (a == c)
			{
                p = 2.0 * xm * s;
                q = 1 - s;
			}
            else
			{
                q = fa / fc;
                r = fb / fc;
                p = s * (2.0 * xm * q * (q - r) - (b - a) * (r - 1.0));
                q = (q - 1.0) * (r - 1.0) * (s - 1.0);
            }
            if (p > 0.0) q = -q;
            p = fabs(p);
            if ((3.0 * xm * q - fabs(tol1 * q)) < fabs(e * q))
			{
                aaa = 3.0 * xm * q - fabs(tol1 * q);
			}
            else
			{
                aaa = fabs(e * q);
            }
            if ((2.0 * p) < aaa)
			{
                e = d;
                d = p / q;
			}
            else
			{
                d = xm;
                e = d;
            }
		}
        else
		{
            d = xm;
            e = d;
        }
        a = b;
        fa = fb;
        if (fabs(d) > tol1)
            b = b + d;
        else
		{
			if (xm>0)
				t=1;
			if (xm==0) 
				t=0;
			if (xm<0) 
				t=-1;
            b = b + fabs(tol1) * t;
		}
        fb = func(b);
	}
	tt=b;
	return tt;
    cout<< "zbrent exceeding maximum iterations."<<endl;
}