x(u,v) = (a+ sgn(cos(u)) |cos(u)|^s) sgn(cos(v)) |cos(v)|^t [br]y(u,v) = (b+ sgn(cos(u)) |cos(u)|^s) sgn(sin(v)) |sin(v)|^t [br]z(u,v) = sgn((sin(u)) |sin(u)|^s