f(x,y) = 5 arcsin(sin(4x)) arcsin(sin(4y))[br]a(t) = cos(2pi t)[br]b(t) = sin(2pi t)[br]r(t) = c k /(1+exp(-f(a(t),b(t)))[br][br]x(t) = a(t) - r(t) b'(t) /sqrt(a'(t)²+b'(t)²)[br]y(t) = b(t) +r(t) a'(t)/sqrt(a'(t)²+b'(t)²)