# Math Help - Operator well-defined

1. ## Operator well-defined

Hello,

I just can't see why the following operator is well-defined: Let $x \in \ell^{2}(\mathbb{N}), a \in \ell^{1}(\mathbb{N})$. Define the operator $A$ so that $Ax = (\sum_{k=1}^{\infty}a_{k+n-1}x_{k})_{n \in \mathbb{N}}$. Also, assume that $a_n$ is monotonically decreasing. Can anybody show me why $A: \ell^{2}(\mathbb{N}) \rightarrow \ell^{2}(\mathbb{N})$ is supposed to be well-defined?

Best regards,
Stiwan

2. ## Re: Operator well-defined

So, we are to prove that

$Ax\in \ell^2(\mathbb{N}), \forall x\in \ell^2(\mathbb{N}).$

This will be true if we can establish that

$||Ax||^2=\sum_n|(Ax)_n|^2<\infty \ \ (1)$

Based on the facts that $a_n\in\ell^1$ and that $a_n$ is decreasing, we can use the Cauchy-Schwartz inequality
to obtain that, eventually
$||Ax||^2=\sum_n\sum_k|a_{k-n+1}x_k|^2\leq (\sum_k|x_k|)^2\sum_n(\sum_k|a_{k-n+1}|) \ \ (2)$

Now, we can use elementary calculus to show that, for large $n\in \mathbb{N}$, we can have
$\sum_l|a_{k-n+1}|\leq n^{-2}$

and so, (2) implies that (1) is true.

3. ## Re: Operator well-defined

I have come up with an answer myself in the meantime, and I don't think that yours is entirely correct.

Originally Posted by Rebesques

$||Ax||^2=\sum_n\sum_k|a_{k-n+1}x_k|^2$
I don't think this is correct, in my opinion it should look like this:

$\|Ax\|_{\ell^{2}}^{2} = \sum_{n=1}^{\infty} \left|\sum_{k=1}^{\infty} a_{k+n-1}x_k \right|^2 \leq \sum_{n=1}^{\infty} \left( \sum_{k=1}^{\infty} |a_{k+n-1}x_k | \right)^2 \leq (\text{Cauchy-Schwarz}) \sum_{n=1}^{\infty} \left(\sum_{k=1}^{\infty}|a_{k+n-1}|^2 \cdot \sum_{k=1}^{\infty}|x_k|^2 \right) = \|x\|_{\ell^{2}}^2 \cdot \sum_{n=1}^{\infty} \sum_{k=1}^{\infty}|a_{k+n-1}|^2$

Now, because of the monotonicity of $(a_n)_{n\in\mathbb{N}}$, we can establish that $|a_{n+k-1}| \leq |a_{k}| \; \forall n,k \in \mathbb{N}$ and in the same way $|a_{n+k-1}| \leq |a_{n}| \; \forall n,k \in \mathbb{N}$. Using this, we get

$\|x\|_{\ell^2}^2 \cdot \sum_{n=1}^{\infty} \sum_{k=1}^{\infty}|a_{k+n-1}|^2 \leq \|x\|_{\ell^2}^2 \cdot \sum_{n=1}^{\infty}\sum_{k=1}^{\infty} |a_n|\cdot |a_k| = \|x\|_{\ell^{2}}^{2} \cdot \|a\|_{\ell^{1}}^{2}$.

Best Regards,
Stiwan

4. ## Re: Operator well-defined

Υes, you are right about the square on $|a_{k-n+1}x_k|$, it was a lapse. And your solution is elegant.

But my gravest errors are these:

1) In the last series. I have written
$\sum_k a_{k-n+1}\leq n^{-2}$

whereas, it should be something more in the like of

$\sum_{k\geq k_n} |a_{k-n+1}|\leq n^{-2}$.

2) The indices of the series are $a_{n+k-1}$ and not my $a_{k-n+1}$, for which one cannot invoke monotonicity save
for the eventual $|a_{k-n+1}|^2\leq |a_{n-k+1}|$.
So actually, i was dealing with a different exercise.... time for a myopia check.