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I recentaly find an article where it is said that there is a sequence of continuous functions $\{f_n:[0,1]\rightarrow\Bbb R\}_{n\in \Bbb N}$ that converges pointqise almost everywhere to zero function , but not converges uniformly.

My question is the following :--

Let $f:[0,1]\rightarrow \Bbb R$ be continuous , does there always exists a sequence of continuous functions $\{f_n:[0,1]\rightarrow\Bbb R\}_{n\in \Bbb N}$ such that $f_n\rightarrow f$ pointwise, but not uniformly on $[0,1]$.

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Yes, there is always such a function.

Take a sequence of continuous functions $g_n$ which converges pointwise, but not uniformly, to $0$. Then $f_n=f + g_n$ converges pointwise, but not uniformly to $f$.

For an example of such $g_n$, you can take the following: $$ g_n(x) = \cases{ nx & if $x< \frac1n$\\ 2-nx & if $\frac1n \leq x < \frac2n$\\ 0& otherwise} $$ The graph of $g_n$ will be a triangle starting at $(0,0)$, going up to $(1, \frac1n)$, then down to $(\frac2n, 0)$, and then flat horizontal from there. This does converge pointwise to $0$ (as at any non-zero point $a\in [0,1]$, eventually $\frac2n<a$, and $g_n(a) = 0$), but not uniformly, as it always has a maximum value of $1$.

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    $\begingroup$ Heh, the exact same answer up to the letter used in order to avoid duplication of $f_n$ :) $\endgroup$ – 5xum Dec 12 '18 at 13:33
  • $\begingroup$ @5xum I gues it's pretty standardized how one makes the "trivial" examples. $\endgroup$ – Arthur Dec 12 '18 at 13:38
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Let $g_n$ be the sequence that converges almost everywhere to $0$, but does not converge uniformly (we already know that exists).

Let $f$ be a continuous function.

Then, $f_n=f+g_n$ converges almost everywhere to $f$, but does not converge uniformly.

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