# 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 {An, φn} where each An is a finite-dimensional full matrix algebra and each φn : AnAn+1 is a unital embedding. Suppressing the connecting maps, one can write

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

## Classification

If

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

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

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

where Ir is the identity in the r × r matrices. The sequence ...kn|kn + 1|kn + 2... determines a formal product

${\displaystyle \delta (A)=\prod _{p}p^{t_{p}}}$

where each p is prime and tp = sup {m   |   pm divides kn for some n}, possibly zero or infinite. The formal product δ(A) is said to be the supernatural number corresponding to A.[1] Glimm showed that the supernatural number is a complete invariant of UHF C*-algebras.[2] In particular, there are uncountably many isomorphism classes of UHF C*-algebras.

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

In the language of K-theory, each supernatural number

${\displaystyle \delta (A)=\prod _{p}p^{t_{p}}}$

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. [1]

## 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 fn and L(H) the bounded operators on H, consider a linear map

${\displaystyle \alpha :H\rightarrow L(H)}$

with the property that

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

The CAR algebra is the C*-algebra generated by

${\displaystyle \{\alpha (f_{n})\}\;.}$

The embedding

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

can be identified with the multiplicity 2 embedding

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

Therefore, the CAR algebra has supernatural number 2.[3] This identification also yields that its K0 group is the dyadic rationals.

## References

1. ^ a b Rørdam, M.; Larsen, F.; Laustsen, N.J. (2000). An Introduction to K-Theory for C*-Algebras. Cambridge: Cambridge University Press. ISBN 0521789443.
2. ^ Glimm, James G. (1 February 1960). "On a certain class of operator algebras" (PDF). Transactions of the American Mathematical Society. 95 (2): 318–340. doi:10.1090/S0002-9947-1960-0112057-5. Retrieved 2 March 2013.
3. ^ Davidson, Kenneth (1997). C*-Algebras by Example. Fields Institute. pp. 166, 218–219, 234. ISBN 0-8218-0599-1.