source: sasmodels/sasmodels/models/lib/gfn.c @ 81dd619

//
//     FUNCTION gfn4:    CONTAINS F(Q,A,B,MU)**2  AS GIVEN
//                       BY (53) & (58-59) IN CHEN AND
//                       KOTLARCHYK REFERENCE
//
//       <OBLATE ELLIPSOID>
// function gfn4 for oblate ellipsoids
double
gfn4(double xx, double crmaj, double crmin, double trmaj, double trmin, double delpc, double delps, double qq)
{
        // local variables
        double aa,bb,u2,ut2,uq,ut,vc,vt,siq,sit,gfnc,gfnt,tgfn,gfn4,pi43,Pi;

        Pi = 4.0*atan(1.0);
        pi43=4.0/3.0*Pi;
        aa = crmaj;
        bb = crmin;
        u2 = (bb*bb*xx*xx + aa*aa*(1.0-xx*xx));
        ut2 = (trmin*trmin*xx*xx + trmaj*trmaj*(1.0-xx*xx));
        uq = sqrt(u2)*qq;
        ut= sqrt(ut2)*qq;
        vc = pi43*aa*aa*bb;
        vt = pi43*trmaj*trmaj*trmin;
        if (uq == 0.0){
                siq = 1.0/3.0;
        }else{
                siq = (sin(uq)/uq/uq - cos(uq)/uq)/uq;
        }
        if (ut == 0.0){
                sit = 1.0/3.0;
        }else{
                sit = (sin(ut)/ut/ut - cos(ut)/ut)/ut;
        }
        gfnc = 3.0*siq*vc*delpc;
        gfnt = 3.0*sit*vt*delps;
        tgfn = gfnc+gfnt;
        gfn4 = tgfn*tgfn;

        return (gfn4);
}