# Help with nested interval proof

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• Nov 5th 2010, 08:06 AM
zebra2147
Help with nested interval proof
The proof I'm working on states:

Let $[a_{n},b_{n}]$ be nested intervals such that $b_{n}-a_{n}$ is null. Let $x_{0}$ be the real number satisfying $\bigcap_{n=1}^\infty [a_{n},b_{n}]=\{x_{0}\}$. Let $\delta >0$. Prove there is an $N$, such that $[a_{N},b_{N}]\subseteq (x_{0}-\delta ,x_{0}+\delta )$.

Here is the direction that I think I need to go but I'm not sure...
Since $\bigcap_{n=1}^\infty [a_{n},b_{n}]=\{x_{0}\}$, and $x_{0}-\delta then there must be an interval $[a_{N},b_{N}]$ that is also contained in $(x_{0}-\delta ,x_{0}+\delta )$. But all that might be completely wrong.
• Nov 5th 2010, 08:26 AM
Plato
Here some useful observations.
$\left( {\forall n} \right)\left[ {a_n \leqslant x_0 \leqslant b_n } \right]$.

$\left( {a_n } \right) \to x_0 \;\& \;\left( {b_n } \right) \to x_0$

If $\delta>0$ find an $\left( N \right)\left[ {x_0 - a_N < \frac{\delta }{2}\;\& \,b_N - x_0 < \frac{\delta }{2}} \right]$
• Nov 5th 2010, 08:27 AM
Drexel28
Quote:

Originally Posted by zebra2147
The proof I'm working on states:

Let $[a_{n},b_{n}]$ be nested intervals such that $b_{n}-a_{n}$ is null. Let $x_{0}$ be the real number satisfying $\bigcap_{n=1}^\infty [a_{n},b_{n}]=\{x_{0}\}$. Let $\delta >0$. Prove there is an $N$, such that $[a_{N},b_{N}]\subseteq (x_{0}-\delta ,x_{0}+\delta )$.

Here is the direction that I think I need to go but I'm not sure...
Since $\bigcap_{n=1}^\infty [a_{n},b_{n}]=\{x_{0}\}$, and $x_{0}-\delta then there must be an interval $[a_{N},b_{N}]$ that is also contained in $(x_{0}-\delta ,x_{0}+\delta )$. But all that might be completely wrong.

Suppose not, then $[a_N,b_N]\supseteq (x_{0}-\delta,x_0+\delta)$ for all $N\in\mathbb{N}$ so that $\displaystyle (x_0-\delta,x_0+\delta)\subseteq\bigcap_{n\in\mathbb{N} }[a_n,b_n]$...but what's wrong with that?
• Nov 6th 2010, 12:50 PM
zebra2147
Well, we stated that $\bigcap_{n\in\mathbb{N}}[a_n,b_n]$ contained only one element, $x_{0}$, but the way you stated it in the last line of your last post, $\bigcap_{n\in\mathbb{N}}[a_n,b_n]$ contains numerous point is the interval $(x_0-\delta,x_0+\delta)$.
• Nov 6th 2010, 01:04 PM
Drexel28
Quote:

Originally Posted by zebra2147
Well, we stated that $\bigcap_{n\in\mathbb{N}}[a_n,b_n]$ contained only one element, $x_{0}$, but the way you stated it in the last line of your last post, $\bigcap_{n\in\mathbb{N}}[a_n,b_n]$ contains numerous point is the interval $(x_0-\delta,x_0+\delta)$.

That's right.