# Is bounded open interval not compact [duplicate]

How to proof that a bounded, open interval is not compact.

• i think you need to write your definition of compactness . Feb 2, 2015 at 23:19

That depends on your definition of compactness.

For $(a,b)$, either consider the open sets $\{(a,b-1/n):n \in \Bbb N\}$ or the sequence $x_n = b - 1/n$ (with $n \in \Bbb N$).

Which space? Do you mean $\mathbb{R}^n$? Recall the definition of compactness in $\mathbb{R}^n$. $$A \subset \mathbb{R}^n \text{is compact, iff} A \text{is closed and bounded}$$ You can show that there is a sequence, such that the limit point is not contained in your open intervall.

• Surely that's not the textbook definition, since that claim is false in general. Feb 2, 2015 at 23:19
• yeah, in genereal. thats why i asked : do you mean $\mathbb{R}^n$. of course i mean $\mathbb{R}^n$ as a metric space.
– aGer
Feb 2, 2015 at 23:25

If $\,I=\,(\,a,b\,)\,$ is such an interval, the sequence of intervals $\,(a,a+\frac1{2^n}]$ is a decreasing sequence of closed intervals of the topological space $I$, and the intersection of all these closed intervals is empty.

Hint: Consider the cover $A=\bigcup_{n\in \mathbb{N}}\{(\frac{1}{n},1)\}$ for the interval $(0,1) \subset \mathbb{R}$.