struct u_exact : field_functor<u_exact,Float> {
  Float operator() (const point& x) const { 
    Float r = sqrt(sqr(x[0])+sqr(x[1])+sqr(x[2]));
    return cos(a*r); }
  u_exact(size_t=0) : a(acos(Float(-1.0))) {}
  Float a;
};
