# Math Help - closed interval continiuous

1. ## closed interval continiuous

Prove the following: Let $a < b$ be real numbers. and let $f: [a,b] \to \bold{R}$ be a function which is continuous on $[a,b]$. Then $f$ is also uniformly continuous.

Assume for contradiction that $f$ is not uniformly continuous. Then there are two equivalent sequences $(x_{n})_{n=0}^{\infty}$ and $(y_{n})_{n=0}^{\infty}$ in $[a,b]$ such that the sequences $(f(x_{n}))_{n=0}^{\infty}$ and $(f(y_{n}))_{n=0}^{\infty}$ are not equivalent. Then we let $E = \{n \in \bold{N}: f(x_{n}) \ \text{and} \ f(y_{n}) \ \text{are not} \ \varepsilon \ \text{close} \}$. Then use Bolzano-Weierstrass Theorem to obtain contradiction?

Is this correct?

2. If $f$ is not uniformly continous then for there is $\epsilon > 0$ such that for any $\delta > 0$ we have $|x-y|<\delta$, $x,y\in [a,b]$ but $|f(x)-f(y)|\geq \epsilon$.
Let $\delta = \frac{1}{n}$ for $n\in \mathbb{Z}^+$.
Then we form two sequences $\{x_n\}$ and $\{y_n\}$ such that $|x_n-y_n|<\tfrac{1}{n}$ and $|f(x_n)-f(y_n)| \geq \epsilon$. Since $\{x_n\}$ is a bounded sequence by Bolzano-Weierstrass theorem there is a convergent subsequence $X_n$. Let $X = \lim ~ X_n$ and note that $X\in [a,b]$. Let $Y_n$ be the subsequence chosen in the same manner as $X_n$ i.e. if $X_n = x_{g(n)}$ for some choice function $g$ then $Y_n = y_{g(n)}$. Now since $|X_n - Y_n|<\tfrac{1}{g(n)}\leq \tfrac{1}{n}$ we see that $X_n - \tfrac{1}{n} < Y_n < X_n + \tfrac{1}{n}$. But each $X_n$ converges to $X$ and so by squeeze-theorem $Y_n$ converges to $X$. Thus, $\{X_n\}$ and $\{Y_n\}$ are two convergent sequences to $X\in [a,b]$. By continuity it means $\{f(X_n)\}$ and $\{f(Y_n)\}$ converge both to $f(X)$. However, $|f(X_n)-f(Y_n)| \geq \epsilon$ and this is a contradiction.

3. And the set $E$ is infinite? Because if it was finite, then there would be two sequences that are equivalent?