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Module Haagerup tensor product

Let $ X$ be a right operator module over a $ C^*$-algebra $ A$, $ Y$ a left $ A$-operator module, and $ W$ an operator space. A bilinear mapping $ \Psi : X \times Y \rightarrow W$ is called balanced, if the equation

$\displaystyle \Psi (x \cdot a, y) = \Psi(x,a \cdot y)$

obtains for all $ x \in X, y \in Y, a \in A$. The module Haagerup tensor product is defined to be the operator space $ X \otimes_{hA} Y$ (which is unique up to complete isometry) together with a bilinear, completely contractive, balanced mapping

$\displaystyle \otimes_{hA} : X \times Y \rightarrow X \otimes_{hA} Y,$

such that the following holds true: For each bilinear, completely bounded balanced map

$\displaystyle \Psi : X \times Y \rightarrow W$

there is a unique linear completely bounded map

$\displaystyle \widetilde{\Psi} : X \otimes_{hA} Y \rightarrow W$

satisfying $ \widetilde{\Psi} \circ \otimes_{hA} = \Psi$ and $ \Vert \widetilde{\Psi} \Vert _{\mathrm{cb}} = \Vert \Psi \Vert _{\mathrm{cb}}$. The module Haagerup tensor product can be realized in different ways:
  1. Let

    $\displaystyle N := \mathrm{lin} \{ (x \cdot a) \otimes y - x \otimes (a \cdot y) \mid a
\in A, x \in X,
y \in Y\}$.

    The quotient space $ (X \otimes_h Y) \big/ \overline{N}$ with its canonical matrix norms is an operator space which satisfies the defintion of $ X \otimes_{hA} Y$ [BMP].
  2. Let us denote by $ X \otimes_A Y$ the algebraic module tensor product, i.e. the quotient space $ (X \otimes_{\mathrm{alg}} Y) \big{/}N$. For $ n\in{\mathbb{N}}$ and $ u \in M_n(X \otimes_A Y)$, by

    $\displaystyle p_n(u) := \inf \left \{ \Vert S\Vert \Vert T\Vert \left\vert
u =...
...ght ],
l \in {\mathbb{N}}, S \in M_{nl}(X), T \in M_{ln}(Y) \right. \right\}
$

    we define a semi-norm on $ M_n( X \otimes_A Y)$. We obtain

    $\displaystyle \mathrm{Kern}(p_n) = M_n(\mathrm{Kern}(p_1)),$

    and the semi-norms $ p_n$ give an operator space norm on $ (X \otimes_A Y) / \mathrm{Kern}(p_1)$ [Rua89]. The completion of this space satisfies the definition of $ X \otimes_{hA} Y$ [BMP].

next up previous contents index
Next: Examples Up: Module tensor products Previous: Module tensor products   Contents   Index
Prof. Gerd Wittstock 2001-01-07