struct u_exact : field_functor<u_exact,Float> {
  Float operator() (const point& x) const {
    return (1 - pow(norm2(x), p/(2*p-2)))/((p/(p-1))*pow(2.,1/(p-1)));
  }
  u_exact (Float q) : p(q) {}
  protected: Float p;
};
struct grad_u : field_functor<grad_u,point> {
  point operator() (const point& x) const {
    return - (pow(norm2(x), p/(2*p-2) - 1)/pow(2.,1/(p-1)))*x;
  }
  grad_u (Float q) : p(q) {}
  protected: Float p;
};
