Tomita–Takesaki theory

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In the theory of von Neumann algebras, a part of the mathematical field of functional analysis, Tomita–Takesaki theory is a method for constructing modular automorphisms of von Neumann algebras from the polar decomposition of a certain involution. It is essential for the theory of type III factors, and has led to a good structure theory for these previously intractable objects.

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The theory was introduced by MinoruTomita  ( 1967 ), but his work was hard to follow and mostly unpublished, and little notice was taken of it until MasamichiTakesaki  ( 1970 ) wrote an account of Tomita's theory.

Modular automorphisms of a state

Suppose that M is a von Neumann algebra acting on a Hilbert space H, and Ω is a cyclic and separating vector of H of norm 1. (Cyclic means that is dense in H, and separating means that the map from M to is injective.) We write for the state of M, so that H is constructed from using the Gelfand–Naimark–Segal construction.

We can define an unbounded antilinear operator S0 on H with domain by setting for all m in M, and similarly we can define an unbounded antilinear operator F0 on H with domain M'Ω by setting for m in M, where M is the commutant of M.

These operators are closable, and we denote their closures by S and F = S*. They have polar decompositions

where is an antilinear isometry called the modular conjugation and is a positive self-adjoint operator called the modular operator.

The main result of Tomita–Takesaki theory states that:

for all t and that

the commutant of M.

There is a 1-parameter family of modular automorphisms of M associated to the state , defined by

The Connes cocycle

The modular automorphism group of a von Neumann algebra M depends on the choice of state φ. Connes discovered that changing the state does not change the image of the modular automorphism in the outer automorphism group of M. More precisely, given two faithful states φ and ψ of M, we can find unitary elements ut of M for all real t such that

so that the modular automorphisms differ by inner automorphisms, and moreover ut satisfies the 1-cocycle condition

In particular, there is a canonical homomorphism from the additive group of reals to the outer automorphism group of M, that is independent of the choice of faithful state.

KMS states

The term KMS state comes from the Kubo–Martin–Schwinger condition in quantum statistical mechanics.

A KMS state φ on a von Neumann algebra M with a given 1-parameter group of automorphisms αt is a state fixed by the automorphisms such that for every pair of elements A, B of M there is a bounded continuous function F in the strip 0 ≤ Im(t) ≤ 1, holomorphic in the interior, such that

Takesaki and Winnink showed that a (faithful semi finite normal) state φ is a KMS state for the 1-parameter group of modular automorphisms . Moreover, this characterizes the modular automorphisms of φ.

(There is often an extra parameter, denoted by β, used in the theory of KMS states. In the description above this has been normalized to be 1 by rescaling the 1-parameter family of automorphisms.)

Structure of type III factors

We have seen above that there is a canonical homomorphism δ from the group of reals to the outer automorphism group of a von Neumann algebra, given by modular automorphisms. The kernel of δ is an important invariant of the algebra. For simplicity assume that the von Neumann algebra is a factor. Then the possibilities for the kernel of δ are:

Hilbert algebras

The main results of Tomita–Takesaki theory were proved using left and right Hilbert algebras.

A left Hilbert algebra is an algebra with involution xx and an inner product (,) such that

  1. Left multiplication by a fixed aA is a bounded operator.
  2. ♯ is the adjoint; in other words (xy, z) = (y, xz).
  3. The involution is preclosed
  4. The subalgebra spanned by all products xy is dense in A.

A right Hilbert algebra is defined similarly (with an involution ♭) with left and right reversed in the conditions above.

A Hilbert algebra is a left Hilbert algebra such that in addition ♯ is an isometry, in other words (x, y) = (y, x).

Examples:

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