2, from the plane geometry analysis:

(1) In the first case:

Gathering point: let C 1 be a point set without boundary, that is, sqrt (x 2+y 2) < r, let any point on the boundary of C 1 be a centripetal neighborhood, and Uo(A, r). No matter how small R is, there is always a point in C2 that belongs to C 1, and this point is called C60.

Boundary point: let C 1 be a point set without boundary, that is, sqrt (x 2+y 2) < r, and let the centripetal neighborhood Uo(A, r) of point a on the boundary of C 1. No matter how small r is, C2 contains points belonging to C 1 and points not belonging to C.

(2) In the second case:

Gathering point: let C 1 be a point set without boundaries, that is, sqrt (x 2+y 2) < r, and let any point a in C 1 have a centripetal neighborhood Uo(A, r). No matter how small R is, no matter how close point A is to the boundary, A is not on the boundary, and C2 always belongs to C6544.

Boundary point: let C 1 be a point set without boundary, that is, sqrt (x 2+y 2) < r, and let any point a in C 1 be a centripetal domain Uo(A, r). No matter how small r is, no matter how close point A is to the boundary, A is not on the boundary. By definition, C2 has nothing that does not belong to C66.