Tuesday 21 April 2015

real analysis - Show the equivalence $x=1$ $⟺$ $-ε

Let $x\in \mathbb{R}$. Show the equivalence $x=1 \Leftrightarrow-\epsilon\lt (x-1)\lt \epsilon$ for all $ε\gt0$.



So the first thing I thought to do was to prove both sides ($\Leftarrow$ and $\Rightarrow$) since this is an equivalence question.



i) To show $\Leftarrow$:




Suppose $x=1$, we have to show $-\epsilon \lt (x-1)\lt \epsilon$ for all $\epsilon\gt0$.



$1
If $x=1$ then $1\lt (1+\epsilon )$, $\epsilon \gt0$



$(x-1)\lt \epsilon$
$(x-\epsilon )\lt 1$



If $x=1$ then $1-ε<1$




Therefore $-\epsilon \lt 0$, so $\epsilon \gt 0$.



Then i would go on to prove the $\Rightarrow$side, but i'm not sure if i'm on the right line or not. Would really appreciate the help.

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