Non-Commutative Integration
  We will show that if $\sM$ is a factor, then for any pair $\f, \p\in\sMdsup$ of normal positive linear functionals on $\sM$, the inequality: $$ \lrnorm{\f}\leq \lrnorm{\p} $$ is equivalent to the fact that there exist a countable family $\lrbrace{\ffdi: i\in I}\subset \sMdsup$ in $\sMdsup$ and a family $\lrbrace{\udi: i\in I}\i\sM$ of partial isometries in \cM such that $$ \f=\sumd{i\in I} \ffdi,\quad \sumd{i\in I} \udi{\ffdi}\udius\leq \p, \quad \text{and} \quad \udius\udi=s\lr{\ffdi}, i\in I, $$ where $s(\omega), \omega\in\sMdsup$, means the support projection of $\omega$. Furthermore, if $\lrnorm{\f}=\lrnorm{\p}$, then the equality replaces the inequality in the second statement. In the case that $\sM$ is not of type \threeonec the family of partial isometries can be replaced by a family of unitaries in \cMp One cannot expect to have this result in the usual integration thoery. To have a similar result, one needs to bring in some kind of non-commutativity. Let $\lrbrace{X, \mu}$ be a $\sig$-finite semifinite measure space and $G$ be an ergodic group of automorphisms of $\linflr{X, \mu}$, then for a pair $f$ and $g$ of $\mu$-integrable positive functions on $X$, the inequality: $$ \int_X f(x)\txd \mu(x)\leq \int_X g(x)\txd \mu(x) $$ is equivalent to the existence of a countable families $\lrbrace{\fdi: i\in I}\subset L^1(X, \mu)$ of positive integrable functions and $\lrbrace{\gdi: i\in I}$ in $G$ such that $$ f=\sumd{i\in I} \fdi\quad\text{and}\quad \sumd{i\in I} \gdi\lr{\fdi}\leq g, $$ where the summation and inequality are all taken in the oredered Banach space $L^1(X, \mu)$ and the action of $G$ on $\lonelr{X, \mu}$ is defined through the duality between $\linflr{X, \mu}$ and $\lonelr{X, \mu}$, i.e.,   \lr{\g(f)}(x)&=f\lr{\g\inv x}\frac{\txd\mu\scirc \g\inv}{\txd\mu}(x), \quad f\in\lonelr{X, \mu}. 
Masamichi Takesaki
math.OA math.FA