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)′.
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)=A′capM=A′capM″=(AcupM′)′,
[{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 M′subseteqA′ and so A″capM=(A′cupM′)′=A″. So as XsubseteqA′, thus A″subseteqX′capM.
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 a−1ba: so A is just linear combinations of bn and a−1bna 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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