# Cuntz algebra

In mathematics, the Cuntz algebra ${\displaystyle {\mathcal {O}}_{n}}$, named after Joachim Cuntz, is the universal C*-algebra generated by n isometries satisfying certain relations.[1] It is the first concrete example of a separable infinite simple C*-algebra.

Every simple infinite C*-algebra contains, for any given n, a subalgebra that has ${\displaystyle {\mathcal {O}}_{n}}$ as quotient.

## Definition and basic properties

Let n ≥ 2 and H be a separable Hilbert space. Consider the C*-algebra ${\displaystyle {\mathcal {A}}}$ generated by a set

${\displaystyle \{S_{i}\}_{i=1}^{n}}$

of isometries (i.e. ${\displaystyle S_{i}^{*}S_{i}=1}$) acting on H satisfying

${\displaystyle \sum _{i=1}^{n}S_{i}S_{i}^{*}=I.}$

Theorem. The concrete C*-algebra ${\displaystyle {\mathcal {A}}}$ is isomorphic to the universal C*-algebra ${\displaystyle {\mathcal {L}}}$ generated by n generators s1... sn subject to relations si*si = 1 for all i and ∑ sisi* = 1.

The proof of the theorem hinges on the following fact: any C*-algebra generated by n isometries s1... sn with orthogonal ranges contains a copy of the UHF algebra ${\displaystyle {\mathcal {F}}}$ type n. Namely ${\displaystyle {\mathcal {F}}}$ is spanned by words of the form

${\displaystyle s_{i_{1}}\cdots s_{i_{k}}s_{j_{1}}^{*}\cdots s_{j_{k}}^{*},k\geq 0.}$

The *-subalgebra ${\displaystyle {\mathcal {F}}}$, being approximately finite-dimensional, has a unique C*-norm. The subalgebra ${\displaystyle {\mathcal {F}}}$ plays role of the space of Fourier coefficients for elements of the algebra. A key technical lemma, due to Cuntz, is that an element in the algebra is zero if and only if all its Fourier coefficients vanish. Using this, one can show that the quotient map from ${\displaystyle {\mathcal {L}}}$ to ${\displaystyle {\mathcal {A}}}$ is injective, which proves the theorem.

This universal C*-algebra is called the Cuntz algebra, denoted by ${\displaystyle {\mathcal {O}}_{n}}$.

A simple C*-algebra is said to be purely infinite if every hereditary C*-subalgebra of it is infinite. ${\displaystyle {\mathcal {O}}_{n}}$ is a separable, simple, purely infinite C*-algebra.

Any simple infinite C*-algebra contains a subalgebra that has ${\displaystyle {\mathcal {O}}_{n}}$ as a quotient.

The UHF algebra ${\displaystyle {\mathcal {F}}}$ has a non-unital subalgebra ${\displaystyle {\mathcal {F}}'}$ that is canonically isomorphic to ${\displaystyle {\mathcal {F}}}$ itself: In the Mn stage of the direct system defining ${\displaystyle {\mathcal {F}}}$, consider the rank-1 projection e11, the matrix that is 1 in the upper left corner and zero elsewhere. Propagate this projection through the direct system. At the Mnk stage of the direct system, one has a rank nk - 1 projection. In the direct limit, this gives a projection P in ${\displaystyle {\mathcal {F}}}$. The corner

${\displaystyle P{\mathcal {F}}P={\mathcal {F'}}}$

is isomorphic to ${\displaystyle {\mathcal {F}}}$. The *-endomorphism Φ that maps ${\displaystyle {\mathcal {F}}}$ onto ${\displaystyle {\mathcal {F}}'}$ is implemented by the isometry s1, i.e. Φ(·) = s1(·)s1*. ${\displaystyle \;{\mathcal {O}}_{n}}$is in fact the crossed product of ${\displaystyle {\mathcal {F}}}$ with the endomorphism Φ.

## Classification

The Cuntz algebras are pairwise non-isomorphic, i.e. ${\displaystyle {\mathcal {O}}_{n}}$ and ${\displaystyle {\mathcal {O}}_{m}}$ are non-isomorphic for nm. The K0 group of ${\displaystyle {\mathcal {O}}_{n}}$ is Zn − 1, the cyclic group of order n − 1. Since K0 is a functor, ${\displaystyle {\mathcal {O}}_{n}}$ and ${\displaystyle {\mathcal {O}}_{m}}$ are non-isomorphic.

## Generalisations

Cuntz algebras have been generalised in many ways. Notable amongst which are the Cuntz–Krieger algebras, graph C*-algebras and k-graph C*-algebras.

## Applied mathematics

In signal processing, subband filter with exact reconstruction give rise to representations of Cuntz algebra. The same filter also comes from the multiresolution analysis construction in wavelet theory.[2]

## References

1. ^ Cuntz, Joachim (1977). "Simple C*-algebras generated by isometries". Comm. Math. Phys. 57. pp. 173–185. Bibcode:1977CMaPh..57..173C. doi:10.1007/bf01625776. Zbl 0399.46045.
2. ^ Jørgensen, Palle E. T.; Treadway, Brian. Analysis and Probability: Wavelets, Signals, Fractals. Graduate Texts in Mathematics. 234. Springer-Verlag. ISBN 0-387-29519-4.