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If $A$ is an element of $\mathbb{R}^n \otimes\mathbb{R}^n \otimes\mathbb{R}^n$, then define its injective tensor norm to be $$\|A\|_{\rm inj} := \max_{x,y,z\in \mathbb{R}^n, \|x\|=\|y\|=\|z\|=1} |\langle A, x\otimes y\otimes z\rangle|.$$ Here the norm on vectors is the usual Euclidean norm.

I have two questions.

  1. Does this norm change if x,y,z can be taken to arbitrary complex unit vectors?
  2. What if $A$ is symmetric under exchange of the first two positions? That is,
    $\langle A, x\otimes y\otimes z\rangle = \langle A, y\otimes x\otimes z\rangle$ for all $x,y,z$.

edit: My motivation for this question is related to the paper https://arxiv.org/abs/1205.4484

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  • $\begingroup$ I'm having trouble getting the $$ equation to work. $\endgroup$
    – aram
    Commented Sep 30, 2011 at 23:01
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    $\begingroup$ Fixed your LaTeX and added the "banach-spaces" tag $\endgroup$
    – Yemon Choi
    Commented Sep 30, 2011 at 23:08
  • $\begingroup$ As clarification: what exactly is the second question? Is it whether the norm changes if $A$ is symmetric in two factors and the vectors are allowed to be complex (as in the first question)? $\endgroup$
    – ARupinski
    Commented Sep 30, 2011 at 23:22
  • $\begingroup$ Yes. Sorry for being unclear. $\endgroup$
    – aram
    Commented Oct 1, 2011 at 0:58

1 Answer 1

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I just found this paper, which gives an example in which the real and complex version of the norm are different. The tensor in this example is also symmetric, which provides an example for part 2 as well.

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