Relative commutants of abelian von Neumann algebras - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T04:05:19Z http://mathoverflow.net/feeds/question/85092 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/85092/relative-commutants-of-abelian-von-neumann-algebras Relative commutants of abelian von Neumann algebras Jesse Peterson 2012-01-06T22:13:41Z 2012-01-06T22:13:41Z <p>This question arose from the discussion over at the question <a href="http://mathoverflow.net/questions/84863/centralizers-in-c-algebra" rel="nofollow">Centralizers in $C^*$-algebras</a>.</p> <p>Which von Neumann algebras $N$ satisfy the property that $A' \cap N = B' \cap N \implies A = B$, for all commutative von Neumann subalgebras $A, B \subset N$?</p> <p>Note that $N = \mathcal B(\mathcal H)$ has this property by von Neumann's double commutant theorem, and perhaps this property characterizes $\mathcal B(\mathcal H)$. It is clear that $N$ must be a factor by considering $A = \mathbb C$ and $B = \mathcal Z(N)$. If $\mathbb F_2 = \langle a, b \rangle$ is the free group on two generators, then by considering the Fourier expansion of elements in $L\mathbb F_2$ it is not hard to see that for $A = L\langle a \rangle$ and $B = L\langle a^2 \rangle$ we have $A' \cap L\mathbb F_2 = B' \cap L\mathbb F_2$ thus $L\mathbb F_2$ does not have this property. </p> <p>Also note that if we were to consider the case when $A$ and $B$ are allowed to be non-commutative then relevant is Corollary 4.1 in Popa's paper <a href="http://www.ams.org/mathscinet-getitem?mr=641131" rel="nofollow">On a Problem of R.V. Kadison on Maximal Abelian $*$-Subalgebras in Factors</a> which shows that every type $II$ factor $N$ contains a hyperfinite subfactor $R$ such that $R' \cap N = \mathbb C$.</p>