Uniformly hyperfinite algebra

In mathematics, particularly in the theory of C*-algebras, a uniformly hyperfinite, or UHF, algebra is a C*-algebra that can be written as the closure, in the norm topology, of an increasing union of finite-dimensional full matrix algebras.

Definition

A UHF C*-algebra is the direct limit of an inductive system {A_n, φ_n} where each A_n is a finite-dimensional full matrix algebra and each φ_n : A_n → A_n+1 is a unital embedding. Suppressing the connecting maps, one can write

$A=\overline{{\cup }_n{A}_n}.$

Classification

If

$A_n\simeq M_{k_n}(ℂ),$

then rk_n = k_{n + 1} for some integer r and

${\varphi }_n(a)=a\otimes I_r,$

where I_r is the identity in the r × r matrices. The sequence ...k_n|k_{n + 1}|k_{n + 2}... determines a formal product

$\delta (A)=\prod _p{p}^{t_p}$

where each p is prime and t_p = sup {m   |   p^m divides k_n for some n}, possibly zero or infinite. The formal product δ(A) is said to be the supernatural number corresponding to A. Glimm showed that the supernatural number is a complete invariant of UHF C*-algebras. In particular, there are uncountably many isomorphism classes of UHF C*-algebras.

If δ(A) is finite, then A is the full matrix algebra M_δ(A). A UHF algebra is said to be of infinite type if each t_p in δ(A) is 0 or ∞.

In the language of K-theory, each supernatural number

$\delta (A)=\prod _p{p}^{t_p}$

specifies an additive subgroup of Q that is the rational numbers of the type n/m where m formally divides δ(A). This group is the K₀ group of A.

CAR algebra

One example of a UHF C*-algebra is the CAR algebra. It is defined as follows: let H be a separable complex Hilbert space H with orthonormal basis f_n and L(H) the bounded operators on H, consider a linear map

$\alpha :H\to L(H)$

with the property that

$\{\alpha (f_n),\alpha (f_m)\}=0\text{and}\alpha (f_n{)}^{*}\alpha (f_m)+\alpha (f_m)\alpha (f_n{)}^{*}=⟨f_m,f_n⟩I.$

The CAR algebra is the C*-algebra generated by

$\{\alpha (f_n)\}.$

The embedding

$C^{*}(\alpha (f_1),\cdots ,\alpha (f_n))\hookrightarrow C^{*}(\alpha (f_1),\cdots ,\alpha (f_{n+1}))$

can be identified with the multiplicity 2 embedding

$M_{2^n}\hookrightarrow M_{2^{n+1}}.$

Therefore, the CAR algebra has supernatural number 2^∞. This identification also yields that its K₀ group is the dyadic rationals.