Are interior points also limit points

In summary, interior points are included in limit points because the definition of interior points requires all points in the ball to be in the set, while the definition of limit points only requires one other point in the ball to be in the set. The set of all limit points of a set is called the closure of the set, which is the union of the interior and boundary of the set. Both limit points and interior points are defined using open balls.
  • #1
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Homework Statement


Are interior points included in (or part of) limit points?

Homework Equations


Since the definition of interior points says that you can find a ball completely contained in the set. For limit points, it's less strict, you just have to find a point other than the center.

The Attempt at a Solution


My guess is yes, but I've never read it anywhere in the book.
 
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  • #2
You should double check on that but I think the set of all limit points of S is called the closure of S, which is the union of the interior of S and the boundary of S, so the short answer would be: YES
 
  • #3
susskind_leon said:
You should double check on that but I think the set of all limit points of S is called the closure of S, which is the union of the interior of S and the boundary of S, so the short answer would be: YES

We use open ball to define both limit point and interior point.

Say, we have some set of space, call it S.

When point p is a limit point of S, we say that we can find a point q (q≠p) in B(x, r), meaning an open ball centered at x, with radius r.

When it is interior point, we say all of the points in this ball is also in S.

So, for limit point, you just need one other point. For interior point, you need all points in the ball is also contained in set S.
 

1. Are interior points and limit points the same thing?

No, interior points and limit points are not the same thing. Interior points are points within a set that have a neighborhood entirely contained within the set. Limit points, on the other hand, are points that can be approached arbitrarily closely by points in the set. In some cases, an interior point can also be a limit point, but this is not always true.

2. Can a set have interior points but no limit points?

Yes, a set can have interior points but no limit points. This occurs when the set is open and does not contain any boundary points. In this case, all interior points are isolated points and cannot be approached by any other points in the set.

3. Are all limit points also interior points?

No, not all limit points are interior points. A limit point can be on the boundary of a set, and therefore not have a neighborhood contained entirely within the set. This means it is not an interior point. However, all interior points are limit points if the set is closed.

4. How do interior points and limit points relate to the concept of closure?

Interior points and limit points are both used to define the closure of a set. The closure of a set is the set itself plus all of its limit points. In other words, the closure contains all the points that the set is "approaching" but may not be included in the set itself. Interior points are not included in the closure because they are already part of the set.

5. Can a set have infinitely many interior points and limit points?

Yes, a set can have infinitely many interior points and limit points. For example, an open interval on the real number line has infinitely many interior points and limit points. However, a discrete set, such as the set of integers, has no interior points or limit points.

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