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In mathematics, the '''spectrum of a [[C*-algebra]]''' or '''dual of a C*-algebra''' ''A'', denoted ''Â'', is the set of [[unitary equivalence]] classes of [[irreducible representation|irreducible]] *-representations of ''A''. A [[*-representation]] π of ''A'' on a [[Hilbert space]] ''H'' is '''irreducible''' if, and only if, there is no closed subspace ''K'' different from ''H'' and {0} which is invariant under all operators π(''x'') with ''x'' ∈ ''A''. We implicitly assume that irreducible representation means ''non-null'' irreducible representation, thus excluding trivial (i.e. identically 0) representations on one-[[dimension]]al [[space (mathematics)|spaces]]. As explained below, the spectrum ''Â'' is also naturally a [[topological space]]; this generalizes the notion of the [[spectrum of a ring]].

One of the most important applications of this concept is to provide a notion of [[duality (mathematics)|dual]] object for any [[locally compact group]]. This dual object is suitable for formulating a [[Fourier transform]] and a [[Plancherel theorem]] for [[unimodular group|unimodular]] [[separable space|separable]] locally compact groups of type I and a decomposition theorem for arbitrary representations of separable locally compact groups of type I. The resulting duality theory for locally compact groups is however much weaker than the [[Tannaka–Krein duality]] theory for [[compact topological group]]s or [[Pontryagin duality]] for locally compact ''abelian'' groups, both of which are complete invariants. That the dual is not a complete invariant is easily seen as the dual of any finite dimensional full matrix algebra M''n''('''C''') consists of a single point.

== Primitive spectrum ==

The [[topology]] of ''Â'' can be defined in several equivalent ways. We first define it in terms of the '''primitive spectrum''' .

The primitive spectrum of ''A'' is the set of [[primitive ideal]]s Prim(''A'') of ''A'', where a primitive ideal is the kernel of an irreducible *-representation. The set of primitive ideals is a [[topological space]] with the '''hull-kernel topology''' (or '''Jacobson topology'''). This is defined as follows: If ''X'' is a set of primitive ideals, its '''hull-kernel closure''' is

:<math> \overline{X} = \{\rho \in \operatorname{Prim}(A): \rho \supseteq \bigcap_{\pi \in X} \pi\}. </math>

Hull-kernel closure is easily shown to be an [[idempotent]] operation, that is

:<math> \overline{\overline{X}} = \overline{X},</math>

and it can be shown to satisfy the [[Kuratowski closure axioms]]. As a consequence, it can be shown that there is a unique topology τ on Prim(''A'') such that the closure of a set ''X'' with respect to τ is identical to the hull-kernel closure of ''X''.

Since unitarily equivalent representations have the same kernel, the map π {{mapsto}} ker(π) factors through a [[surjective]] map

:<math> \operatorname{k}: \hat{A} \rightarrow \operatorname{Prim}(A). </math>

We use the map ''k'' to define the topology on ''Â'' as follows:

'''Definition'''. The open sets of ''Â'' are inverse images ''k''−1(''U'') of open subsets ''U'' of Prim(''A''). This is indeed a topology.

The hull-kernel topology is an analogue for non-commutative rings of the [[Zariski topology]] for commutative rings.

The topology on ''Â'' induced from the hull-kernel topology has other characterizations in terms of [[state (functional analysis)|state]]s of ''A''.

== Examples ==

=== Commutative C*-algebras ===
[[File:3-dim commut algebra, subalgebras, ideals.svg|thumb|left|224px|3-dimensional commutative C*-algebra and its ideals. Each of 8 ideals corresponds to a closed subset of discrete 3-points space (or to an open complement). Primitive ideals correspond to closed [[singleton (mathematics)|singletons]]. See details at the image description page.]]
The spectrum of a commutative C*-algebra ''A'' coincides with the [[Gelfand transformation|usual dual]] of ''A'' (not to be confused with the [[Banach space|dual]] ''A''' of the Banach space ''A''). In particular, suppose ''X'' is a [[compact space|compact]] [[Hausdorff space]]. Then there is a [[natural transformation|natural]] [[homeomorphism]]

:<math> \operatorname{I}: X \cong \operatorname{Prim}( \operatorname{C}(X)).</math>

This mapping is defined by

: <math> \operatorname{I}(x) = \{f \in \operatorname{C}(X): f(x) = 0 \}.</math>

I(''x'') is a closed maximal ideal in C(''X'') so is in fact primitive. For details of the proof, see the Dixmier reference. For a commutative C*-algebra,

:<math> \hat{A} \cong \operatorname{Prim}(A).</math>

=== The C*-algebra of bounded operators ===

Let ''H'' be a separable [[Hilbert space]]. ''L''(''H'') has two norm-closed *-ideals: ''I''0 = {0} and the ideal ''K'' = ''K''(''H'') of compact operators. Thus as a set, Prim(''L''(''H'')) = {''I''0, ''K''}. Now

* {''K''} is a closed subset of Prim(''L''(''H'')).

* The closure of {''I''0} is Prim(''L''(''H'')).

Thus Prim(''L''(''H'')) is a non-Hausdorff space.

The spectrum of ''L''(''H'') on the other hand is much larger. There are many inequivalent irreducible representations with kernel ''K''(''H'') or with kernel {0}.

=== Finite dimensional C*-algebras ===

Suppose ''A'' is a finite dimensional C*-algebra. It is known ''A'' is isomorphic to a finite direct sum of full matrix algebras:

:<math> A \cong \bigoplus_{e \in \operatorname{min}(A)} A e, </math>

where min(''A'') are the minimal central projections of ''A''. The spectrum of ''A'' is canonically isomorphic to min(''A'') with the [[discrete topology]]. For finite dimensional C*-algebras, we also have the isomorphism

:<math> \hat{A} \cong \operatorname{Prim}(A).</math>

== Other characterizations of the spectrum ==

The hull-kernel topology is easy to describe abstractly, but in practice for C*-algebras associated to [[locally compact]] [[topological group]]s, other characterizations of the topology on the spectrum in terms of positive definite functions are desirable.

In fact, the topology on ''Â'' is intimately connected with the concept of [[weak containment]] of representations as is shown by the following:

'''Theorem'''. Let ''S'' be subset of ''Â''. Then the following are equivalent for an irreducile representation π

# The equivalence class of π in ''Â'' is in the closure of ''S''
# Every state associated to π, that is one of the form

::<math> f_\xi(x) = \langle \xi \mid \pi(x) \xi \rangle </math>

:with ||ξ||=1, is the weak limit of states associated to representations in ''S''.

The second condition means exactly that π is weakly contained in ''S''.

The [[GNS construction]] is a recipe for associating states of a C*-algebra ''A'' to representations of ''A''. By one of the basic theorems associated to the GNS construction, a state ''f'' is [[pure state|pure]] if and only if the associated representation π''f'' is irreducible. Moreover, the mapping κ: PureState(''A'') → ''Â'' defined by ''f'' {{mapsto}} π''f'' is a surjective map.

From the previous theorem one can easily prove the following;

'''Theorem''' The mapping

: <math> \kappa: \operatorname{PureState}(A) \rightarrow \hat{A} </math>

given by the GNS construction is continuous and open.

=== The space Irr''n''(''A'') ===

There is yet another characterization of the topology on ''Â'' which arises by considering the space of representations as a topological space with an appropriate pointwise convergence topology. More precisely, let ''n'' be a cardinal number and let ''H''''n'' be the canonical Hilbert space of dimension ''n''.

Irr''n''(''A'') is the space of irreducible *-representations of ''A'' on ''H''''n'' with the point-weak topology. In terms of convergence of nets, this topology is defined by π''i'' → π if and only if

:<math> \langle \pi_i(x) \xi \mid \eta \rangle \rightarrow \langle \pi(x) \xi \mid \eta \rangle \quad \forall \xi, \eta \in H_n \ x \in A. </math>

It turns out that this topology on Irr''n''(''A'') is the same as the point-strong topology, i.e. π''i'' → π if and only if

:<math> \pi_i(x) \xi \rightarrow \pi(x) \xi \quad \mbox{ normwise } \forall \xi \in H_n \ x \in A. </math>

'''Theorem'''. Let ''Â''''n'' be the subset of ''Â'' consisting of equivalence classes of representations whose underlying Hilbert space has dimension ''n''. The canonical map Irr''n''(''A'') → ''Â''''n'' is continuous and open. In particular, ''Â''''n'' can be regarded as the quotient topological space of Irr''n''(''A'') under unitary equivalence.

'''Remark'''. The piecing together of the various ''Â''''n'' can be quite complicated.

== Mackey Borel structure ==

''Â'' is a topological space and thus can also be regarded as a [[Borel set|Borel space]]. A famous conjecture of [[G. Mackey]] proposed that a ''separable'' locally compact group is of type I if and only if the Borel space is standard, i.e. is isomorphic (in the category of Borel spaces) to the underlying Borel space of a [[Polish space|complete separable metric space]]. Mackey called Borel spaces with this property '''smooth'''. This conjecture was proved by [[James Glimm]] for separable C*-algebras in the 1961 paper listed in the references below.

'''Definition'''. A non-degenerate *-representation π of a separable C*-algebra ''A'' is a '''factor representation''' if and only if the center of the von Neumann algebra generated by π(''A'') is one-dimensional. A C*-algebra ''A'' is of type I if and only if any separable factor representation of ''A'' is a finite or countable multiple of an irreducible one.

Examples of separable locally compact groups ''G'' such that C*(''G'') is of type I are [[connected space|connected]] (real) [[nilpotent]] [[Lie group]]s and connected real [[semi-simple]] Lie groups. Thus the [[Heisenberg group]]s are all of type I. Compact and abelian groups are also of type I.

'''Theorem'''. If ''A'' is separable, ''Â'' is smooth if and only if ''A'' is of type I.

The result implies a far-reaching generalization of the structure of representations of separable type I C*-algebras and correspondingly of separable locally compact groups of type I.

== Algebraic primitive spectra ==

Since a C*-algebra ''A'' is a [[ring (mathematics)|ring]], we can also consider the set of [[primitive ideal]]s of ''A'', where ''A'' is regarded algebraically. For a ring an ideal is primitive if and only if it is the [[Annihilator (ring theory)|annihilator]] of a [[simple module]]. It turns out that for a C*-algebra ''A'', an ideal is algebraically primitive [[if and only if]] it is primitive in the sense defined above.

'''Theorem'''. Let ''A'' be a C*-algebra. Any algebraically irreducible representation of ''A'' on a complex vector space is algebraically equivalent to a topologically irreducible *-representation on a Hilbert space. Topologically irreducible *-representations on a Hilbert space are algebraically isomorphic if and only if they are unitarily equivalent.

This is the Corollary of Theorem 2.9.5 of the Dixmier reference.

If ''G'' is a locally compact group, the topology on dual space of the [[group algebra|group C*-algebra]] C*(''G'') of ''G'' is called the '''Fell topology''', named after [[J. M. G. Fell]].

== References ==
* J. Dixmier, ''Les C*-algèbres et leurs représentations'', Gauthier-Villars, 1969.
* J. Glimm, ''Type I C*-algebras'', Annals of Mathematics, vol 73, 1961.
* G. Mackey, ''The Theory of Group Representations'', The University of Chicago Press, 1955.

{{Functional Analysis}}

[[Category:C*-algebras]]
[[Category:Spectral theory]]

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