double form_volume(double length, double kuhn_length, double radius);
double Iq(double q, double length, double kuhn_length, double radius,
          double sld, double solvent_sld);
double Iqxy(double qx, double qy, double length, double kuhn_length,
            double radius, double sld, double solvent_sld);
double flexible_cylinder_kernel(double q, double length, double kuhn_length,
                                double radius, double sld, double solvent_sld);


double form_volume(double length, double kuhn_length, double radius)
{

      return 0.0;
}

double flexible_cylinder_kernel(double q,
          double length,
          double kuhn_length,
          double radius,
          double sld,
          double solvent_sld)
{

	double Pi = 4.0*atan(1.0);

	double cont = sld-solvent_sld;
	double qr = q*radius;
	double flex = Sk_WR(q,length,kuhn_length);
    double crossSect = (2.0*J1(qr)/qr)*(2.0*J1(qr)/qr);

	flex *= crossSect;
	flex *= Pi*radius*radius*length;
	flex *= cont*cont;
	flex *= 1.0e-4;

	return flex;
}

double Iq(double q,
          double length,
          double kuhn_length,
          double radius,
          double sld,
          double solvent_sld)
{

	double result = flexible_cylinder_kernel(q, length, kuhn_length, radius, sld, solvent_sld);

	return result;
}

double Iqxy(double qx, double qy,
            double length,
            double kuhn_length,
            double radius,
            double sld,
            double solvent_sld)
{
	double q;
	q = sqrt(qx*qx+qy*qy);
	double result = flexible_cylinder_kernel(q, length, kuhn_length, radius, sld, solvent_sld);

	return result;
}