Subfactor

In the theory of von Neumann algebras, a subfactor of a factor M is a subalgebra that is a factor and contains 1. The theory of subfactors led to the discovery of the Jones polynomial in knot theory.

Index of a subfactor

Usually M is taken to be a factor of type II₁, so that it has a finite trace. In this case every Hilbert space module H has a dimension dim_M(H) which is a non-negative real number or +∞. The index [M:N] of a subfactor N is defined to be dim_N(L²(M)). Here L²(M) is the representation of N obtained from the GNS construction of the trace of M.

The Jones index theorem

This states that if N is a subfactor of M (both of type II₁) then the index [M : N] is either of the form 4 cos(π/n)² for n = 3, 4, 5, ..., or is at least 4. All these values occur.

The first few values of 4 cos(π/n)² are 1, 2, (3 + √5)/2 = 2.618..., 3, 3.247..., ...

The basic construction

Suppose that N is a subfactor of M, and that both are finite von Neumann algebras. The GNS construction produces a Hilbert space L²(M) acted on by M with a cyclic vector Ω. Let e_N be the projection onto the subspace NΩ. Then M and e_N generate a new von Neumann algebra <M, e_N> acting on L²(M), containing M as a subfactor. The passage from the inclusion of N in M to the inclusion of M in <M, e_N> is called the basic construction.

If N and M are both factors of type II₁ and N has finite index in M then <M, e_N> is also of type II₁. Moreover the inclusions have the same index: [M:N] = [<M, e_N> :M], and tr_{<M, eN>}(e_N) = 1/[M:N].

The tower

Suppose that M₋₁ ⊆ M₀ is an inclusion of type II₁ factors of finite index. By iterating the basic construction we get a tower of inclusions

M₋₁ ⊆ M₀ ⊆ M₁ ⊆ M₂ ...

where each M_n+1 = <M_n, e_n+1> is generated by the previous algebra and a projection. The union of all these algebras has a tracial state tr whose restriction to each M_n is the tracial state, and so the closure of the union is another type II₁ von Neumann algebra M_∞.

The algebra M_∞ contains a sequence of projections e₁,e₂, e₃,..., which satisfy the Temperley–Lieb relations at parameter λ = 1/[M : N]. Moreover, the algebra generated by the e_n is a C*-algebra in which the e_n are self-adjoint, and such that tr(xe_n) = λ tr(x) when x is in the algebra generated by e₁ up to e_n−1. Whenever these extra conditions are satisfied, the algebra is called a Temperly–Lieb–Jones algebra at parameter λ. It can be shown to be unique up to *-isomorphism. It exists only when λ takes on those special values 4 cos(π/n)² for n = 3, 4, 5, ..., or the values larger than 4.

Principal graphs

A subfactor of finite index N $\subseteq$ M is said to be irreducible if either of the following equivalent conditions is satisfied:

• L²(M) is irreducible as an (N, M) bimodule;
• the relative commutant N ' $\cap$ M is C.

In this case L²(M) defines an (N, M) bimodule X as well as its conjugate (M, N) bimodule X*. The relative tensor product, described in Jones (1983) and often called Connes fusion after a prior definition for general von Neumann algebras of Alain Connes, can be used to define new bimodules over (N, M), (M, N), (M, M) and (N, N) by decomposing the following tensor products into irreducible components:

$X\boxtimes X^* \boxtimes \cdots \boxtimes X,\,\, X^*\boxtimes X \boxtimes \cdots \boxtimes X^*, \,\, X^* \boxtimes X \boxtimes \cdots \boxtimes X,\,\, X\boxtimes X^* \boxtimes \cdots \boxtimes X^*.$

The irreducible (M, M) and (M, N) bimodules arising in this way form the vertices of the principal graph, a bipartite graph. The directed edges of these graphs describe the way an irreducible bimodule decomposes when tensored with X and X* on the right. The dual principal graph is defined in a similar way using (N, N) and (N, M) bimodules.

Since any bimodule corresponds to the commuting actions of two factors, each factor is contained in the commutant of the other and therefore defines a subfactor. When the bimodule is irreducible, its dimension is defined to be the square root of the index of this subfactor. The dimension is extended additively to direct sums of irreducible bimodules. It is multiplicative with respect to Connes fusion.

The subfactor is said to have finite depth if the principal graph and its dual are finite, i.e. if only finitely many irreducible bimodules occur in these decompositions. In this case if M and N are hyperfinite, Sorin Popa showed that the inclusion N $\subseteq$ M is isomorphic to the model

$(\mathbf{C}\otimes \mathrm{End}\, X^*\boxtimes X \boxtimes X^*\boxtimes \cdots)^{\prime\prime} \subseteq (\mathrm{End}\, X\boxtimes X^* \boxtimes X \boxtimes X^* \boxtimes\cdots )^{\prime\prime},$

where the II₁ factors are obtained from the GNS construction with respect to the canonical trace.

Knot polynomials

The algebra generated by the elements e_n with the relations above is called the Temperley–Lieb algebra. This is a quotient of the group algebra of the braid group, so representations of the Temperley–Lieb algebra give representations of the braid group, which in turn often give invariants for knots.

References

• Jones, V.F.R. (1983), "Index for subfactors", Invent. Math. 72: 1–25, doi:10.1007/BF01389127
• Wenzl, H.G. (1988), "Hecke algebras of type A_n and subfactors", Invent. Math. 92 (2): 349–383, doi:10.1007/BF01404457
• V. Jones, V. S. Sunder, Introduction to subfactors, ISBN 0-521-58420-5
• Theory of Operator Algebras III by M. Takesaki ISBN 3-540-42913-1
• A. J. Wassermann, Operators on Hilbert space