function F = Hpresfunc_ho(v,x0,p0,h)
  global k m
  x1 = v(1);
  p1 = v(2);
  F(1) = (x1-x0)*(p1-p0) - h*(H_ho(x0,p1)-H_ho(x0,p0));
  F(2) = (p1-p0)*(x1-x0) + h*(H_ho(x1,p1)-H_ho(x0,p1));
end
