Definition of Limit Point: Is There a Point Not in E?

In summary, a limit point p is a point in a subset E of a metric space X where every neighborhood of p contains a point q≠p in E. It is possible for there to be a limit point p that is not in E. An example of a set that is perfect is the closed interval [0,1]. A closed set that is not perfect can have an isolated point, such as {2} or [0,1]\cup{2}.
  • #1
jwqwerty
43
0
the definition of limit point:
a point p is a limit point of E(subset of metric space X) if every neighborhood of p contains a point q≠p which is in E.

My question is that is there a limit point p which is not in E?
 
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  • #2
jwqwerty said:
the definition of limit point:
a point p is a limit point of E(subset of metric space X) if every neighborhood of p contains a point q≠p which is in E.

My question is that is there a limit point p which is not in E?

Take the open unit interval E = (0,1) as a subset of the real numbers. Can you think of a real number that's not in (0,1) but that satisfies the definition of limit point of E? (Hint: Can you think of TWO such points?)
 
  • #3
SteveL27 said:
Take the open unit interval E = (0,1) as a subset of the real numbers. Can you think of a real number that's not in (0,1) but that satisfies the definition of limit point of E? (Hint: Can you think of TWO such points?)

thanks but i have another question

can you give me an example of a set that is perfect?
def: E is perfect if E is closed and if every point of E is a limit point of E
 
  • #4
So you refuse to answer SteveL27's question?

But I will answer your question: [0, 1].

It's actually harder to give an example of a closed set that is NOT perfect. Can you?
 
  • #5
HallsofIvy said:
It's actually harder to give an example of a closed set that is NOT perfect. Can you?

What stops me from adding {0} to usual topology of real line, so that is´s open set? Then (0, 1] would be closed and not perfect. Certainly not easy, I can´t think of any more standard example.
 
  • #6
(0, 1] is not closed; it's just also not open. A good example of a closed non-perfect set is one with an isolated point, like {2}, or [0,1][itex]\cup[/itex]{2}.
 

1. What is a limit point?

A limit point is a point in a set of numbers or points that can be approximated by other points in the set. In other words, it is a point where the points in the set get closer and closer to as they approach the limit point.

2. What is the definition of a limit point?

The formal definition of a limit point is: Let E be a subset of a metric space (X,d). A point x in X is a limit point of E if every neighborhood of x contains at least one point of E different from x itself.

3. How is a limit point different from a cluster point?

A limit point is a point that can be approximated by other points in the set, whereas a cluster point is a point where there is an infinite number of points in the set that are arbitrarily close to it. In other words, a limit point is a specific point, while a cluster point is a set of points.

4. Is there always a limit point in a set?

No, not all sets have limit points. Some sets may not have any points that can be approximated by other points in the set, while others may have an infinite number of limit points.

5. Can a limit point be outside of the set?

Yes, a limit point can be outside of the set. This means that the set does not contain the limit point, but the limit point can still be approximated by points in the set.

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