It is essentially the same as the example in the quotient Green's formula of higher mathematics textbook, except that the textbook is the application of Green's formula, and its condition is partial continuity. Because there is no continuity, a small circle is made. This classic example has real questions in 2009 in all major math review reference books, and its conditions are still partially continuous by using Gauss theorem, so it is obvious that the given integral function is discontinuous at the origin, so a small circle is added. Actually, the essence is the same. Think about it carefully and pay attention to the direction of the small sphere.