Operator algebra

Noncommutative geometry

Index theory



Abstract C *C^\ast-algebras


A C *C^*-algebra is a Banach algebra (A,)(A, {\|-\|}) over a topological field KK (often the field KK \coloneqq \mathbb{C} of complex numbers) equipped with an involution () *(-)^\ast compatible with complex conjugation if appropriate (that is: a Banach star-algebra) that satisfies the C *C^*-identity

A *A=A *A {\|{A^* A}\|} = {\|{A^*}\|} \, {\|{A}\|}

or equivalently the B *B^*-identity

A *A=A 2. {\|{A^* A}\|} = {\|{A}\|^2} \,.

A homomorphism of C *C^\ast-algebras is a map that preserves all this structure. For this it is sufficient for it to be a star-algebra homomorphisms.

C *C^\ast-algebras with these homomorphisms form a category C*Alg.


Often one sees the definition without the clause (which should be in the definition of Banach **-algebra) that the involution is an isometry (so that A *=A{\|A^*\|} = {\|A\|}, which is key for the equivalence of the B *B^* and C *C^* identities). This follows easily from the B *B^*-identity, while it follows from the C *C^*-identity after some difficulty.


There are different concepts for the tensor product of C *C^*-algebras, see for example at spatial tensor product.


C *C^*-algebras equipped with the action of a group by automorphisms of the algebra are called C-star-systems .

Concrete C *C^\ast-algebras and C *C^\ast-representations


Given a complex Hilbert space HH, a concrete C *C^*-algebra on HH is a **-subalgebra of the algebra of bounded operators on HH that is closed in the norm topology.


A representation of a C *C^*-algebra AA on a Hilbert space HH is a **-homomorphism from AA to the algebra of bounded operators on HH.


It is immediate that concrete C *C^*-algebras correspond precisely to faithful representations of abstract C *C^*-algebras. It is an important theorem that every C *C^*-algebra has a faithful representation; that is, every abstract C *C^*-algebra is isomorphic to a concrete C *C^*-algebra.


The original definition of the term ‘C *C^*-algebra’ was in fact the concrete notion; the ‘C’ stood for ‘closed’. Furthermore, the original term for the abstract notion was ‘B *B^*-algebra’ (where the ‘B’ stood for ‘Banach’). However, we now usually interpret ‘C *C^*-algebra’ abstractly. (Compare ‘W *W^*-algebra’ and ‘von Neumann algebra’.)

In \dagger-compact categories

The notion of C *C^*-algebra can be abstracted to the general context of symmetric monoidal †-categories, which serves to illuminate their role in quantum mechanics in terms of †-compact categories.

For a discussion of this in the finite-dimensional case see for instance (Vicary).


C *C^*-algebras are monadic over sets. More precisely, the forgetful functor C *AlgSet\mathbf{C^*Alg}\to\mathbf{Set} that assigns to each algebra the set of points in its unit ball is monadic. See J Wick Pelletier & J Rosicky, On the equational theory of C*C*-algebras, Algebra Universalis 30:275-284, 1993.

See also operator algebras.

Gelfand-Naimark theorem

The Gelfand-Naimark theorem says that every C*-algebra is isomorphic to a C *C^\ast-algebra of bounded linear operators on a Hilbert space. In other words, every abstract C *C^*-algebra may be made into a concrete C *C^*-algebra.

Gelfand-Naimark-Segal construction

The Gelfand-Naimark-Segal construction (GNS construction) establishes a correspondence between cyclic **-representations of C *C^*-algebras and certain linear functionals (usually called states) on those same C *C^*-algebras. The correspondence comes about from an explicit construction of the *-representation from one of the linear functionals (states).

Gelfand duality

Gelfand duality says that every (unital) commutative C *C^*-algebra over the complex numbers is that of complex-valued continuous functions from some compact Hausdorff topological space: there is an equivalence of categories C *CAlg comC^* CAlg_{com} \simeq Top cpt{}_{cpt}.

Accordingly one may think of the study of non-commutative C *C^\ast-algebras as non-commutative topology.



For AA and BB two C *C^\ast-algebras and f:ABf : A \to B a star-algebra homomorphism the set-theoretic image f(A)Bf(A) \subset B is a C *C^\ast-subalgebra of BB, hence is also the image of ff in C *AlgC^\ast Alg.

This is (KadisonRingrose, theorem 4.1.9).


There is a functor

𝒞:C *AlgPoset \mathcal{C} : C^\ast Alg \to Poset

to the category Poset of posets, which sends each AC *AlgA \in C^\ast Alg to its poset of commutative subalgebras 𝒞(A)\mathcal{C}(A) and sends each morphism f:ABf : A \to B to the functor 𝒞(f):𝒞(A)𝒞(B)\mathcal{C}(f) : \mathcal{C}(A) \to \mathcal{C}(B) which sends a commutative subalgebra CAC \subset A to f(C)Bf(C) \subset B.

Construction as groupoid convolution algebras

Many C *C^\ast-algebras arise as groupoid algebras of Lie groupoids. See at groupoid algebra - References - For smooth geometry

Homotopy theory

There is homotopy theory of C *C^\ast-algebras, being a non-commutative generalization of that of Top. (e.g. Uuye 12). For more see at homotopical structure on C*-algebras.



Any algebra M n(A)M_n(A) of matrices with coefficients in a C *C^\ast-algebra is again a C *C^\ast-algebra. In particular M n()M_n(\mathbb{C}) is a C *C^\ast-algebra for all nn \in \mathbb{N}.


For AA a C *C^\ast-algebra and for XX a locally compact Hausdorff topological space, the set of continuous functions XAX \to A which vanish at infinity is again a C *C^\ast-algebra by extending all operations pointwise. (This algebra is unital precisely if AA is and if XX is a compact topological space.)

This algebra is denoted

C 0(X,A)C *Alg. C_0(X,A) \in C^\ast Alg \,.

If A=A = \mathbb{C} then one usually just writes

C 0(X)C 0(X,). C_0(X) \coloneqq C_0(X, \mathbb{C}) \,.

This are the C *C^\ast-algebras to which the Gelfand duality theorem applies.


A uniformly hyperfinite algebra is in particular a C *C^\ast-algebra, by definition.


A von Neumann algebra is in particular a C *C^\ast-algebra, by definition.


A standard textbook reference is chapter 4 in volume 1 of

  • Richard Kadison, John Ringrose, Fundamentals of the theory of operator algebras Academic Press, (1983)

An exposition that explicitly gives Gelfand duality as an equivalence of categories and introduces all the notions of category theory necessary for this statement is in

  • Ivo Dell’Ambrogio, Categories of C *C^\ast-algebras (pdf)

For operator algebra-theory see there and see

A characterizations of injections of commutative sub-C *C^*-algebras – hence of the poset of commutative subalgebras of a C *C^*-algebra – is in

General properties of the category of C *C^\ast-algebras are discussed in

Specifically pullback and pushout of C *C^\ast-algebras is discussed in

  • Gerd Petersen, Pullback and pushout constructions in C *C^\ast-algebra theory (pdf)

The homotopy theory of C *C^\ast-algebras (a category of fibrant objects-structure on C *AlgC^\ast Alg) is discussed in

  • Otgonbayar Uuye, Homotopy theory for C *C^\ast-algebras (arXiv:1011.2926)

For more along such lines see the references at KK-theory and E-theory.

Revised on February 15, 2017 15:53:40 by Toby Bartels (