Classification: education/science >> further studies >> taking the postgraduate entrance examination.

Problem description:

I mainly want to know when the spot is not a "gathering point"

Analysis:

Gathering point: Let x0 be a point (which may or may not belong to point set E), if any centripetal neighborhood n (x0,&; At least one point x belongs to e, then x0 is called a gathering point of e.

Inner points are aggregation points, boundary points are aggregation points, and isolated points are not aggregation points.