Answer to Question #264596 in Real Analysis for Nikhil

Question #264596

The set { x∈ R : x≤ 5} is not a closed set in R.


True or false

1
Expert's answer
2021-11-12T08:20:21-0500

Let us show that the set O={xR:x>5}O=\{ x∈ \R : x> 5\} is open set in R\R. Let aOa\in O be arbitrary, and let εa=a5>0.\varepsilon_a=a-5>0. Then the open ball Bεa(a)=(aεa,a+εa)B_{\varepsilon_a}(a)=(a-\varepsilon_a,a+\varepsilon_a) is contained in O.O. It follows that each point aOa\in O is inner, and hence the set OO is open. Therefore, the set {xR:x5}=RO\{ x∈ \R : x≤ 5\}=\R\setminus O is a closed set in R\R.


Answer: false


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