## Banach Journal of Mathematical Analysis

### Noncommutative integration

Masamichi Takesaki

#### Abstract

We will show that if $\mathcal{M}$ is a factor, then for any pair $\varphi ,p\in\mathcal{M}_*^+$ of normal positive linear functionals on $\mathcal{M}$, the inequality: $$\|\varphi\|\leq \|\psi\|$$ is equivalent to the fact that there exist a countable family $\{\varphi_i : i\in I\}\subset \mathcal{M}_*^+$ in $\mathcal{M}_*^+$ and a family $\{u_i : i\in I\}\subset\mathcal{M}$ of partial isometries in $\mathcal{M}$ such that $$\varphi=\sum_{ i\in I} \varphi_i,\quad \sum_{ i\in I} u_i{\varphi_i}u_i^*\leq \psi, \quad \text{and} \quad u_i^*u_i=s(\varphi_i), i\in I,$$ where $s(\omega), \omega\in \mathcal{M}_*^+$, means the support projection of $\omega$. Furthermore, if $\|\varphi\|=\|\psi\|$, then the equality replaces the inequality in the second statement. In the case that $\mathcal{M}$ is not of type III$_1$, the family of partial isometries can be replaced by a family of unitaries in $\mathcal{M}$. One cannot expect to have this result in the usual integration theory. To have a similar result, one needs to bring in some kind of non-commutativity. Let $\{X, \mu\}$ be a $\sigma$-finite semifinite measure space and $G$ be an ergodic group of automorphisms of $L^\infty(X,\mu)$, then for a pair $f$ and $g$ of $\mu$-integrable positive functions on $X$, the inequality: $$\int_X f(x)\text{d} \mu(x)\leq \int_X g(x)\text{d} \mu(x)$$ is equivalent to the existence of a countable families $\{f_i: i\in I\}\subset L^1(X, \mu)$ of positive integrable functions and $\{\gamma_i: i\in I\}$ in $G$ such that $$f=\sum_{ i\in I} f_i\quad\text{and}\quad \sum_{ i\in I} \gamma_i(f_i)\leq g,$$ where the summation and inequality are all taken in the ordered Banach space $L^1(X, \mu)$ and the action of $G$ on $L^1(X, \mu)$ is defined through the duality between $L^\infty(X, \mu)$ and $L^1(X, \mu)$, i.e., $$(\gamma(f))(x)=f(\gamma^{-1} x)\frac{d\mu\circ \gamma^{-1}}{d\mu}(x), \quad f\in L^1(X, \mu).$$

#### Article information

Source
Banach J. Math. Anal., Volume 7, Number 1 (2013), 1-13.

Dates
First available in Project Euclid: 22 January 2013

https://projecteuclid.org/euclid.bjma/1358864545

Digital Object Identifier
doi:10.15352/bjma/1358864545

Mathematical Reviews number (MathSciNet)
MR3004263

Zentralblatt MATH identifier
1292.46042

Subjects
Primary: 46L10: General theory of von Neumann algebras
Secondary: 46L05: General theory of $C^*$-algebras

#### Citation

Takesaki, Masamichi. Noncommutative integration. Banach J. Math. Anal. 7 (2013), no. 1, 1--13. doi:10.15352/bjma/1358864545. https://projecteuclid.org/euclid.bjma/1358864545

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