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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 Quote by jwqwerty 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?)

 Quote by SteveL27 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

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So you refuse to answer SteveL27's question?

 (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]$\cup${2}.