\( X(u,v)=(R+r\cos v\cos u,R+r\cos v\cos u\sin v,r\sin v) \)

 

with \( R=2, \) \( r=1.2, \) \( 0\le u,v\le 2\pi \)

\( X(u,v)=(R+ar\cos v\cos u,R+a r\cos v\cos u\sin v,ar\sin v) \)

 

with \( R=2, \) \( r=1.9, \) \( 0\le u,v\le 2\pi, \) and \[ a(u)=-\,\frac{1}{5\pi^4}\,x^4+\frac{4}{5\pi^3}\,x^3-\frac{4}{5\pi^2}\,x^2+\frac{3}{10} \]

\( X(u,v)=(R+ar\cos v\cos u,R+a r\cos v\cos u\sin v,ar\sin v) \)

 

with \( R=2, \) \( r=1.2, \) \( 0\le u\le 8, \) \( 0\le v\le 2\pi, \) and \( a(u)=\cos(0.18u) \)