source: sasmodels/sasmodels/models/hollow_cylinder.c @ 6cf1cb3

double _hollow_cylinder_kernel(double q, double core_radius, double radius, 
        double length, double dum);

double form_volume(double radius, double core_radius, double length);
double Iq(double q, double radius, double core_radius, double length, double sld,
        double solvent_sld);
double Iqxy(double qx, double qy, double radius, double core_radius, double length, double sld,
        double solvent_sld, double theta, double phi);

// From Igor library
double _hollow_cylinder_kernel(double q, double core_radius, double radius, 
        double length, double dum)
{
    double gamma,arg1,arg2,lam1,lam2,psi,sinarg,t2,retval;              //local variables
    
    gamma = core_radius/radius;
    arg1 = q*radius*sqrt(1.0-dum*dum);          //1=shell (outer radius)
    arg2 = q*core_radius*sqrt(1.0-dum*dum);                     //2=core (inner radius)
    if (arg1 == 0.0){
        lam1 = 1.0;
    }else{
        lam1 = 2.0*J1(arg1)/arg1;
    }
    if (arg2 == 0.0){
        lam2 = 1.0;
    }else{
        lam2 = 2.0*J1(arg2)/arg2;
    }
    //Todo: Need to check psi behavior as gamma goes to 1.
    psi = (lam1 -  gamma*gamma*lam2)/(1.0-gamma*gamma);         //SRK 10/19/00
    sinarg = q*length*dum/2.0;
    if (sinarg == 0.0){
        t2 = 1.0;
    }else{
        t2 = sin(sinarg)/sinarg;
    }

    retval = psi*psi*t2*t2;
    
    return(retval);
}

double form_volume(double radius, double core_radius, double length)
{
        double pi = 4.0*atan(1.0);
        double v_shell = pi*length*(radius*radius-core_radius*core_radius);
        return(v_shell);
}

double Iq(double q, double radius, double core_radius, double length, double sld,
        double solvent_sld)
{
    int i;
        int nord=76;                    //order of integration
        double lower,upper,zi, inter;           //upper and lower integration limits
        double summ,answer,delrho;                      //running tally of integration
        double norm,scale,volume,convert;       //final calculation variables
        
        delrho = solvent_sld - sld;
        lower = 0.0;
        upper = 1.0;            //limits of numerical integral

        summ = 0.0;                     //initialize intergral
        for(i=0;i<nord;i++) {
                zi = ( Gauss76Z[i] * (upper-lower) + lower + upper )/2.0;
                inter = Gauss76Wt[i] * _hollow_cylinder_kernel(q, core_radius, radius, length, zi);
                summ += inter;
        }
        
        norm = summ*(upper-lower)/2.0;
        // Multiply by contrast^2
        scale = delrho*delrho;
        //normalize by volume
        volume = form_volume(radius, core_radius, length);
        //convert to [cm-1] given sld*1e6
        convert = 1.0e-4;
        answer = norm*scale*convert*volume*volume;
        
        return(answer);
}

//TODO: Add this in
double Iqxy(double qx, double qy, double radius, double core_radius, double length, double sld,
        double solvent_sld, double theta, double phi)
{
    return(0.0);
}