Uniformly hyperfinite algebra

In operator algebras, a uniformly hyperfinite, or UHF, algebra is one that is the closure, in the appropriate topology, of an increasing union of finite dimensional full matrix algebras.

C*-algebras

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

$A={\overline {\cup _{n}A_{n}}}.$

If

$A_{n}\simeq M_{k_{n}}(\mathbb {C} ),$

then r k_n = k_n + 1 for some integer r and

$\phi _{n}(a)=a\otimes I_{r},$

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 R that is the rational numbers of the type n/m where m formally divides δ(A). This group is the K₀ group of A.

An example

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

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

with the property that

$\{\alpha (f_{n}),\alpha (f_{m})\}=0\quad {\mbox{and}}\quad \alpha (f_{n})^{*}\alpha (f_{m})+\alpha (f_{m})\alpha (f_{n})^{*}=\langle f_{m},f_{n}\rangle I.$

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.