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Let $f:\mathbb{R}\to\mathbb{R}$ be continuous on [0, 1]; show that:

$$ \lim_{p \to +\infty} \left( \int_{0}^{1} |f|^{p} \right)^{\frac{1}{p}}=max|f| $$

My work:

I was able to show that:

$$ \left( \int_{0}^{1} |f|^{p} \right)^{\frac{1}{p}}\le \left( \int_{0}^{1} max(|f|)^{p} \right)^{\frac{1}{p}}=max|f| $$

with $ max|f|\in \mathbb{R} $ due to continuity and Weierstrass.

We can also see that, considering, for every x in [0, 1], $(f(x), 0)\in \mathbb{R}^{2}$:

$$ |f(x)|^{p}=\left( \left\| (f(x), 0) \right\|_{p} \right)^{p} $$

And there exists positive constants c, C such that:

$$ c \cdot max(f, 0)^{p}\le \left( \left\| (f(x), 0) \right\|_{p} \right)^{p}\le C\cdot max(f, 0)^{p} $$

I was thinking of applying Holder/Minkowski to bound the limit from below but I don't know how to continue.

iki
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    This has been asked and answered before – FShrike Jun 09 '23 at 17:29
  • https://math.stackexchange.com/q/3583256/42969, https://math.stackexchange.com/q/4109156/42969, https://math.stackexchange.com/q/1438426/42969 – Martin R Jun 09 '23 at 17:31
  • thanks, I'll give it a look – iki Jun 09 '23 at 17:33
  • In https://math.stackexchange.com/questions/1438426/limits-of-the-p-norm why can we conclude that lim→±∞()=sup{():∈[0,1]} if f is continuous? – iki Jun 09 '23 at 17:56
  • The idea is to fix $0<\varepsilon < \max|f|$ and show that $|f| \geq \max|f| - \varepsilon$ on some nonempty $(a,b) \subset [0,1]$. The only case this doesn't cover is $\max|f|= 0$, which is simple to prove. – Brian Moehring Jun 09 '23 at 18:48

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