{(Cx2 + Cy2 + Cz2) (a2 + b2 + Sb2)}2 = 4a2{b2 C ( sbab )2} (B.7) Sab If we replace Cx, Cy and Cz by x, y and z, respectively, sb by s and ab by a, we will get the same equation as shown in Eq. (3.1), which represents the general form of torus [51].