On genuine infinite algebraic tensor products

Abstract

A genuine infinite tensor product of complex vector spaces is a vector space ${\bigotimes}_{i\in I} X_i$ whose linear maps coincide with multilinear maps on an infinite family $\{X_i\}_{i\in I}$ of vector spaces. We give a direct sum decomposition of ${\bigotimes}_{i\in I} X_i$ over a set $\Omega_{I;X}$, through which we obtain a more concrete description and some properties of ${\bigotimes}_{i\in I} X_i$. If $\{A_i\}_{i\in I}$ is a family of unital $^*$-algebras, we define, through a subgroup $\Omega^{\rm ut}_{I;A}\subseteq \Omega_{I;A}$, an interesting subalgebra ${\bigotimes}_{i\in I}^{\rm ut} A_i$. Moreover, it is shown that ${\bigotimes}_{i\in I}^{\rm ut} \mathbb{C}$ is the group algebra of $\Omega^{\rm ut}_{I;\mathbb{C}}$. In general, ${\bigotimes}_{i\in I}^{\rm ut} A_i$ can be identified with the algebraic crossed product of a cocycle twisted action of $\Omega^{\rm ut}_{I;A}$. If $\{H_i\}_{i\in I}$ is a family of inner-product spaces, we define a Hilbert $C^*(\Omega^{\rm ut}_{I;\mathbb{C}})$-module $\bar\bigotimes^{\rm mod}_{i\in I} H_i$, which is the completion of a subspace ${\bigotimes}_{i\in I}^{\rm unit} H_i$ of ${\bigotimes}_{i\in I} H_i$. If $\chi_{\Omega^{\rm ut}_{I;\mathbb{C}}}$ is the canonical tracial state on $C^*(\Omega^{\rm ut}_{I;\mathbb{C}})$, then $\bar\bigotimes^{\rm mod}_{i\in I} H_i\otimes_{\chi_{\Omega^{\rm ut}_{I;\mathbb{C}}}}\mathbb{C}$ is a natural dilation of the infinite direct product $\prod {{\otimes}_{i\in I}} H_i$ as defined by J. von Neumann. We will show that the canonical representation of ${\bigotimes}_{i\in I}^{\rm ut} \mathcal{L}(H_i)$ on $\bar\bigotimes^{\phi_1}_{i\in I} H_i$ is injective. We will also show that if $\{A_i\}_{i\in I}$ is a family of unital Hilbert algebras, then so is ${\bigotimes}_{i\in I}^{\rm ut} A_i$.