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Let $E$ be a complex Hilbert space, and $\mathcal{L}(E)$ be the algebra of all bounded linear operators on $E$.

For ${\bf A}:=(A_1,...,A_n) \in \mathcal{L}(E)^n$ we recall the definitions of the following two norms on $\mathcal{L}(E)^n$: $$\|{\bf A}\|=\displaystyle\sup_{\|x\|=1}\bigg(\displaystyle\sum_{k=1}^n\|A_kx\|^2\bigg)^{\frac{1}{2}},$$

and $$\omega_e({\bf A})=\displaystyle\sup_{\|x\|=1}\bigg(\displaystyle\sum_{k=1}^n|\langle A_kx\;|\;x\rangle|^2\bigg)^{1/2}.$$

It's not difficult to prove that $\omega_e({\bf A}) \leq \|{\bf A}\|$. Are $\|\cdot\|$ and $\omega_e(\cdot)$ two equivalent norms on $\mathcal{L}(E)^n\,?$ If the answer is true, I hope to find $\alpha$ such that $$\alpha \|{\bf A}\|\leq \omega_e({\bf A}) \leq \|{\bf A}\|.$$ Note that if $n=1$, it is well known that $$\displaystyle\frac{1}{2}\|A\|\leq \omega(A)\leq\|A\|.$$

And you for you help.

Student
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1 Answers1

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Yes, these two norms are equivalent. Recall that the following two norms are equivalent:

$$ \Vert \cdot \Vert_{\max}: \mathbb{R}^n \rightarrow \mathbb{R}, \ \Vert (x_1, \dots, x_n )\Vert_{\max} = \max_{j\in \{1, \dots, n \}} \vert x_j \vert $$

and

$$ \Vert \cdot \Vert_{2}: \mathbb{R}^n \rightarrow \mathbb{R}, \ \Vert (x_1, \dots, x_n )\Vert_{2} = \left( \sum_{j=1}^n \vert x_j \vert^2 \right)^{1/2}.$$

In fact, we have that for all $y\in \mathbb{R}^n$ holds (this are the optimal constants)

$$ \Vert y \Vert_{\max} \leq \Vert y \Vert_2 \leq \sqrt{n} \Vert y \Vert_{\max}.$$

Note that

$$ \Vert A \Vert = \sup_{\Vert x \Vert = 1} \Vert (A_1x, \dots , A_n x)\Vert_2 $$

and

$$ \omega_e(A) = \sup_{\Vert x \Vert = 1} \Vert (\vert \langle A_1 x,x\rangle \vert \dots , \vert \langle A_n x, x \rangle\vert )\Vert_2 $$

Furthermore, as pointed out by @Student we have

$$ \Vert A \Vert \leq 2 \sup_{\Vert x \Vert =1} \vert \langle Ax , x \rangle \vert .$$

This follows from the fact that for normal operators $B$ we have $\Vert B \Vert = \sup_{\Vert x \Vert=1} \vert \langle Bx, x \rangle \vert$ and the following computation

$$ \Vert A \Vert \leq \frac{1}{2} ( \Vert A + A^\star \Vert + \Vert A - A^\star \Vert ) = \frac{1}{2} ( \sup_{\Vert x \Vert = 1} \vert \langle ( A + A^\star)x, x \rangle \vert + \sup_{\Vert x \Vert = 1} \vert \langle ( A - A^\star)x, x \rangle \vert) \leq 2 \sup_{\Vert x \Vert = 1} \vert \langle Ax, x \rangle \vert.$$

Putting everything together yields

$$ \Vert A \Vert = \sup_{\Vert x \Vert = 1} \Vert (A_1x, \dots , A_n x)\Vert_2 \leq \sqrt{n} \sup_{\Vert x \Vert = 1} \Vert (A_1x, \dots , A_n x)\Vert_{\max} \leq 2 \sqrt{n} \sup_{\Vert x \Vert = 1} \Vert (\vert \langle A_1x, x \rangle \vert, \dots , \vert \langle A_n x, x \rangle \vert)\Vert_{\max} \leq 2 \sqrt{n} \sup_{\Vert x \Vert = 1} \Vert (\vert \langle A_1x, x \rangle \vert, \dots , \vert \langle A_n x, x \rangle \vert)\Vert_{2} = 2\sqrt{n} \omega_e(A). $$

Therefore, $\alpha= \frac{1}{2\sqrt{n}}$ will do the job.

Added: I was asked to adress the optimality of the constants in the inequality. The estimate $\omega_e(A) \leq \Vert A \Vert$ is sharp, as we can take $A=(Id, \dots, Id)$ and for this choice we get equality.

The other estimate is more difficult. I only managed to prove optimality under the additional assumption $dim_\mathbb{C}(E)\geq n+1$. In case someone has some insight for lower dimension please let me know.

If we assume $dim_\mathbb{C}(E)\geq n+1$, then it suffices to consider the case $E=\mathbb{C}^{n+1}$ with the standard scalar product (otherwise choose a subspace of dimension $n+1$ and identify it with $\mathbb{C}^{n+1}$). We choose $A_k: \mathbb{C}^{n+1} \rightarrow \mathbb{C}^{n+1}$ such that $$ A_k (x_1, \dots, x_{n+1})= (0, \dots,0 , x_1, 0, \dots, 0) $$ where $x_1$ is in the kth slot. The corresponding matrix consists of all zeros and a one in the kth row of the first column, e.g. for $n=3, k=2$ $$ A_2 = \begin{pmatrix} 0 & 0 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 0 \end{pmatrix}.$$ Now we set $$ A=(A_2, \dots, A_{n+1} )$$ We have $$ \Vert A_k x \Vert^2 = \langle (0, \dots, x_1, \dots, 0),(0, \dots, x_1, \dots, 0)\rangle = \vert x_1 \vert^2 $$ Thus, we get $$ \Vert A \Vert = \sup_{\Vert x \Vert = 1} \sqrt{n} \vert x_1 \vert = \sqrt{n} $$ On the other hand we have $$ \vert \langle A_k x, x \rangle \vert^2 = \vert \langle (0, \dots, x_1, \dots, 0), (x_1, \dots, x_{n+1}) \rangle \vert^2 = \vert x_1 \vert^2 \cdot \vert x_k \vert^2 $$ And hence, for $\Vert x \Vert=1$ $$ \left(\sum_{k=2}^{n+1} \vert \langle A_k x, x \rangle \vert^2\right)^\frac{1}{2} = \left(\vert x_1 \vert^2 \cdot \sum_{k=2}^{n+1} \vert x_k \vert^2 \right)^\frac{1}{2} = \left(\vert x_1 \vert^2 \cdot (1- \vert x_1 \vert^2)\right)^\frac{1}{2} = \vert x_1 \vert \cdot \sqrt{1- \vert x_1 \vert^2}$$ Hence, we get $$ \omega_e(A) = \sup_{\Vert x \Vert=1} \vert x_1 \vert \cdot \sqrt{1- \vert x_1 \vert^2} = \sup_{\vert x_1 \vert\leq 1} \vert x_1 \vert \cdot \sqrt{1- \vert x_1 \vert^2} = \frac{1}{2} $$ Thus, we finally get $$ \frac{1}{2\sqrt{n}} \Vert A \Vert = \frac{1}{2\sqrt{n}} \cdot \sqrt{n} = \frac{1}{2} = \omega_e(A) $$

  • @Student I added some details. Is it fine or should I elaborate a bit more? – Severin Schraven Dec 20 '17 at 11:06
  • @Student Thank you for pointing out my mistakes. I'll correct them as soon as possible. – Severin Schraven Dec 21 '17 at 11:38
  • Dear Professor Severin. I think that since the inequalities $$ \Vert y \Vert_{\max} \leq \Vert y \Vert_2 \leq \sqrt{n} \Vert y \Vert_{\max},$$ are sharp, i.e. $1,\sqrt{n}$ are the optimal constants, then the inequalities $$\frac{1}{2\sqrt{n}}|\mathbf{A}|\leq \omega_e(\mathbf{A})\leq |\mathbf{A}|,$$ are sharp (the constants $\frac{1}{2\sqrt{n}}$ and $1$ are the best). Do you agree with me? Thanks a lot. – Student Jan 10 '19 at 10:15
  • I think it depends on what you mean by optimal. If you are talking about optimality when we want to have the inequalities for all possible operators, then I am optimistic, that we can prove such a thing. Let me think about it. However, if you are looking for optimal constants for a fixed operator, then I think this will become very difficult as we cannot use the optimality of $$ \Vert y \Vert_{max} \leq \Vert y \Vert_2 \leq \Vert y \Vert_{max} $$ anymore (as the operator might not be surjective). For example if we just take the zero operator, our constants are not optimal :) – Severin Schraven Jan 10 '19 at 18:57
  • Please do not adress me with Professor. I am just a PhD-student :) – Severin Schraven Jan 10 '19 at 18:58
  • Thank you very much for your comments and I hope that you become a Professor. I mean by optimal that is the best possible constants. Please see my related question which I discover a mistake in the given answer: https://math.stackexchange.com/questions/2600913/numerical-radius-of-a-pair-of-operators-in-hilbert-spaces/2608982#2608982 – Student Jan 10 '19 at 19:07
  • I see, so you want optimal constants such that the inequality holds for all operators. So we need to find operators such that we get equalities. The 1 is certainly optimal in that case as we can just choose $A_k = Id$ for all $k$. For the other constant I really need to think more. The problem is that we use several estimates and even though $$ \Vert y \Vert_{2} \leq \sqrt{n} \Vert y \Vert_{max}$$ is sharp. It is not clear that the chain of inequalities truely is sharp. In case you make some progress yourself, I'd be happy to hear from you. – Severin Schraven Jan 10 '19 at 20:46
  • Anyway, my feeling is, that we might do better in finite dimension (but I don't know how much), but in infinite dimension we should at least get the factor $\frac{1}{\sqrt{n}}$. My idea is to use isometries such that their images are all orthogonal to each other (this we cannot do in finite dimension). However, I have to do the computation tomorrow, now I have to sleep. – Severin Schraven Jan 10 '19 at 21:46
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    Dear Severin, I would like to thank you for your effort in order to help me. Since a period I'm trying to find an example. According to ([arXiv][1], page 25), if we take $A_k=\frac{1}{\sqrt{n}}T$ for all $k\in {1,\cdots,n}$ with $$T=\begin{pmatrix}0&0\1&0\end{pmatrix}.$$ [1]: https://arxiv.org/pdf/math/0410492.pdf – Student Jan 11 '19 at 06:05
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    We obtain: Hence, $$w(A_1,\cdots,A_n)=\sqrt{n}w(\frac{1}{\sqrt{n}}T)=w(T)=\frac{1}{2},$$ however, $$\displaystyle\frac{1}{2\sqrt{n}}\left|\displaystyle\sum_{k=1}^nA_k^*A_k \right|^{1/2}=\frac{1}{2\sqrt{n}}\sqrt{n}|\frac{1}{\sqrt{n}}T|=\frac{1}{2\sqrt{n}}.$$ – Student Jan 11 '19 at 06:06
  • Very cool! That is a super nice example. Maybe we can tweak it slightly to get a our factor of 2. – Severin Schraven Jan 11 '19 at 08:06
  • I think I can prove that the constant is sharp if we assume that $dim(E)>n$. It suffices to show it for $E= \mathbb{C}^{n+1}$. The trick is to use a variant of your example. Namely we want to define $A_j$ to be the matrix that consists of zeros and a single one in the jth colume of the first row and we take $A=(A_2, \dots, A_{n+1}$. The omission of $A_1$ will give us the missing factor of 2. I will write down the details, when I am awake again. – Severin Schraven Jan 12 '19 at 02:46
  • It is now fine, up to the additional assumption on the dimension. But I have honestly no idea how to deal with lower dimensions. – Severin Schraven Jan 12 '19 at 13:12
  • Thanks a lot. For the benefits of the reader, I think it will be really good if you also copy your example here: https://math.stackexchange.com/questions/3053909/find-an-example-which-shows-that-the-following-inequality-is-sharp – Student Jan 12 '19 at 13:51
  • I did as you suggested. – Severin Schraven Jan 12 '19 at 13:59
  • Thank you very much and I'm really happy to discuss with you. – Student Jan 12 '19 at 14:02
  • It was fun to play around with optimal constants. Even though it is not really my field of expertise. Let's hope someone will solve the low dimensional case. – Severin Schraven Jan 12 '19 at 14:03