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Suppose $X$ is a real number such that $X > 0$. We want to show there exists and $n \in \mathbb{N}$ such that $X \geq \frac{1}{n} $.

MY attempt: If $X < \frac{1}{n} \; \; \; \forall n $ then $X \leq 0 $ by passing to the limit. Contradiction. Can someone show me a way to show this from scratch? By actually finding an $n$ that does this job? thanks

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The Archimedianity of $\Bbb R$ can be seen as a corollary of the unboundedness of $\Bbb N$ in $\Bbb R$ with the usual order. That is, pick $x\in\Bbb R_{>0}$. Then $x^{-1}>0$ is not an upper bound for $\Bbb N$, so there is $n\in\Bbb N$ such that $x^{-1}<n$. Inverting, gives $x>n^{-1}$.

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why cant we have $\geq $ ? –  Chasky Oct 27 '13 at 23:24
    
If $x^{-1}\leqslant n$ then $x^{-1}<n+1$. Thus, there is no harm in putting $<$ directly. –  Pedro Tamaroff Oct 27 '13 at 23:26
    
gotta wait $8$ minutes to accept the answer . sorry –  Chasky Oct 27 '13 at 23:27
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"Unboundedness of $\mathbb N$" is one way to say it, but really you're talking about a property of both $\mathbb N$ and $\mathbb R$. There are some other ordered sets containing $\mathbb N$ in which $\mathbb N$ ** is ** bounded. –  Robert Israel Oct 27 '13 at 23:28
    
thanks for your time che and for your answer –  Chasky Oct 27 '13 at 23:29
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The Archimedian Property of $\mathbb{R}$ can also be described as if $a,b \in \mathbb{R}$, $a,b > 0$, then $\exists n \in \mathbb{N}$ s.t. $na > b$.

Proof:

If $a>b$, let $n=1$. If $a=b$, let $n=2$. If $a<b$, then take the set $X = \{ na | n \in \mathbb{N}\}$. $X$ is nonempty as $a \in X$. Now suppose $X$ has an upper bound, call it $b$. We know from the Least Upper Bound property that any subset of $\mathbb{R}$ that is bounded above has a least upper bound. So let $L = \text{lub}(X)$. Then, since $a>0$, $\exists n_0a \in X$ s.t. $L-a < n_0a$. It follows that $L < (n_0 +1)a$, which is a contradiction.

It is a corollary that for any $\epsilon >0 \in \mathbb{R}$, $\exists n \in \mathbb{N}$ s.t. $\frac{1}{n} < \epsilon$.

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The archimedan property simply states that there are no infinitesimals or infinite numbers in your system/group/set.

For example, you will always find a rational in between two numbers or a large natural after the maximum of your two numbers.

The set $\mathbb{R}$ is archimedan. ($\mathbb{Q}$ is dense in $\mathbb{R}$).

If the rationals can be embedded into a set such that it is dense within the set, it is archimedan.

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