Friday, 21 January 2011

oa.operator algebras - Relative Bicommutant

Let AsubseteqB(H) be a subset, and let textalg(A) be the algebra generated by A. Then it's easy to see that A=textalg(A).

A similarly easy check shows that if A and B are subsets, then AcapB=(AcupB)=textalg(AcupB).



So, for your question, pick some normal representation MsubseteqB(H) (so that M=M), and let AsubseteqM be a subset. Set X=xinM:ax=xa(ainA)=AcapM=AcapM=(AcupM),

so your relative commmutatant is [{min M:xm=mx (xin X) } = X' cap M = X' cap M'' = (Xcup M')'.] So, yes, this is a von Neumann algebra.



In fact, as AsubseteqM, clearly MsubseteqA and so AcapM=(AcupM)=A. So as XsubseteqA, thus AsubseteqXcapM.



Edit: There is probably an easier example than this... But, let M=VN(mathbbF2), say with canonical generators a and b. Let A be a star-algebra generated by b and a1ba: so A is just linear combinations of bn and a1bna for ninmathbbZ. A bit of combinatorics shows that X=mathbbC1 and so the the relative bicommutant is all of M. However, A is not ultraweakly dense in M, because we cannot approximate the generator a.



Conclude: So, if I haven't messed up, this shows that the relative bicommutant is always a W*-subalgebra of M, but that it might be larger than the ultraweak closure of A in M.

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