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Let $\{f_n\}$ be a sequence of functions in $L^\infty$. I want to prove that $\{f_n\}$ converges to $f\in L^\infty$ $\Leftrightarrow$ there is a set $E$ of measure zero such that $f_n$ converges uniformly to $f$ on $E^c$.

**My Attempt:**

$(\Rightarrow)$ Suppose $f_n\rightarrow f$ in $L^\infty$. Then $ \Vert f_n-f\Vert_\infty=\inf \{M:|f_n-f|\leqslant M ~~\text{a.e.}\}\lt \varepsilon$ for any $\varepsilon \gt 0$. Let $E=\{x:|f_n(x)-f(x)|\gt \varepsilon\}.$ The $m(E)=0$. So $f_n\rightarrow f$ uniformly on $E^c$.

$(\Leftarrow)$ Suppose $f_n\rightarrow f$ uniformly on $E^c$ with $m(E)=0$. Then $\forall~t\in E^c$ and $n\gt N$, $|f_n(t)-f(t)|\lt \varepsilon$. But

$$|f(t)|=|f(t)-f_n(t)+f_n(t)|\leqslant |f(t)-f_n(t)|+|f_n(t)|\lt \varepsilon + \Vert f_n(t)\Vert_\infty.$$ So $f$ is bounded and hence $f\in L^\infty$.

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Please, could someone look over what I’ve done and point out any errors?

Thanks.

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Watch your quantifiers.

For the forward direction, unwinding the definitions, you need to show: there exists a set $E$ with $m(E)=0$ such that for every $\epsilon > 0$, there exists $N$ such that for all $n > N$ and all $x \in E^c$, we have $|f_n(x) – f(x)| < \epsilon$. Your set $E$ depends on $n$, so this is not good. Remember that “$f_n \to f$” is a statement about the *sequence* $\{f_n\}$, not a statement about $f_1$ or $f_6$ or any other particular function from the sequence.

So start with what you know: $||f_n – f||_{L^\infty} \to 0$. The quantity $||f_n – f||_{L^\infty}$ bounds $|f_n(x) – f(x)|$ for “most” $x$. That is, if $E_n = \{x : |f_n(x) – f(x)| > ||f_n – f||_{L^\infty}\}$, what can you say about $E_n$? Can you use the sets $E_n$ to come up with a single set $E$ that works for the proof?

For the backward direction, you not only need to show that $f \in L^\infty$, but that $f_n \to f$ in $L^\infty$ norm, i.e. $||f_n – f||_{L^\infty} \to 0$. This is not very hard if you just look at the definition of the $L^\infty$ norm.

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