# Is my proof of this operator norm correct?

Let $$\alpha \in \ell^\infty$$ and $$T_\alpha:\ell^p \rightarrow \ell^p$$ $$(1\leq p \leq \infty)$$ given by $$$$T_\alpha(x)=(\alpha_1x_1,\dots,\alpha_nx_n,\dots).$$$$ Note that $$||T_\alpha(x)||\leq ||\alpha|| \ ||x||$$, thus $$T_\alpha(x) \in \ell^p$$.

Now, I want to prove that $$T_\alpha$$ is continuous and $$||T_\alpha||=\alpha$$.

So, let $$(x_n)=(x_1^n,x_2^n,\dots)\rightarrow(0,0,\dots)\in \ell^p$$. Then $$||x_n||\rightarrow 0$$ and by the above inequallity $$||T_\alpha(x)||\rightarrow 0$$. Then we get that $$T_\alpha$$ is continuous.

Computing the norm, I went for this idea:

$$$$\begin{split} ||\alpha||&=\sup_{||x||=1 \\\ \ x\neq 0} ||\alpha|| \ ||x|| \\ &= \sup_{||x||=1 \\\ \ x\neq 0} \sup_{i \in \mathbb{N}} |\alpha_i| \ ||x|| \\ &=\sup_{||x||=1 \\\ \ x\neq 0} \sup_{i \in \mathbb{N}} |\alpha_i| \ \big(\sum_{j=1}^\infty |x_i|^p \big)^{1/p} \\ &=\sup_{i \in \mathbb{N}}\sup_{||x||=1 \\\ \ x\neq 0} \big(\sum_{j=1}^\infty |\alpha_i|^p|x_i|^p \big)^{1/p} = \sup_{i \in \mathbb{N}} ||T_\alpha||=||T_\alpha||. \end{split}$$$$

Then, in both cases when $$p$$ is finite or $$p=\infty$$ we have that $$||T_\alpha||=||\alpha||$$. Is this proof correct? What I'm missing here? Thanks for sharing!

## 1 Answer

Your transition from the third to the fourth line needs remediation, specifically with your indexing. Your double $$\sup$$'s are also a bit hard to follow. Let me offer your a suggestion.

Define $$A,B\subseteq l^p$$ by $$A=\{e_i\in l^p:i\in \mathbb{N}\}$$ $$B=\{x\in l^p:\|x\|_p=1 \}$$ Because $$A \subseteq B$$ and $$\{|\alpha_i|:i\in \mathbb{N}\}=\Big\{\|T_{\alpha}(x)\|_p:x\in A\Big\}$$ we can say that $$\|\alpha\|_{\infty}=\sup_{i\in \mathbb{N}}|\alpha_i|=\sup_{x\in A}\|T_{\alpha}(x)\|_p\leq \sup_{x\in B}\|T_{\alpha}(x)\|_p=\|T_{\alpha}\|_{\mathcal{L}(\ell^p,\ell^p)}$$

This proves $$\|\alpha\|_{\infty}\leq\|T_{\alpha}\|_{\mathcal{L}(\ell^p,\ell^p)}$$. You can use your inequality $$\|T_{\alpha}(x)\|_p\leq \|x\|_p\|\alpha\|_{\infty}$$ to show $$\|\alpha\|_{\infty}\geq \|T_{\alpha}\|_{\mathcal{L}(\ell^p,\ell^p)}$$.