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| + | * The use of $H$ for Hilbert space and $L(H)$ for the bounded operators on it seems a bit dated, perhaps this should be $\mathscr{H}$ and $\mathscr{B}(\mathscr{H})$, respectively. |
| − | {{TEX|want}}
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| − | In the process of being TeXed
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| − | $$
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| − | \newcommand{\abs}[1]{\left|#1\right|}
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| − | \newcommand{\norm}[1]{\left\|#1\right\|}
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| − | \newcommand{\set}[1]{\left\{#1\right\}}
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| − | \newcommand{\Ah}{A_{\text{h}}}
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| − | $$
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| − | A
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| − | [[Banach algebra|Banach algebra]] $A$ over the field of complex numbers, with an involution $x \rightarrow x^*$, $x \in A$, such that the norm and the involution are connected by the relation $\norm{x^* x} = \norm{x}^2$ for any element $x \in A$. $C^*$-algebras were introduced in 1943
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| − | {{Cite|GeNe}} under the name of totally regular rings; they are also known under the name of $B^*$-algebras. The most important examples of $C^*$-algebras are:
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| − | 1) The algebra $C_0(X)$ of continuous complex-valued functions on a locally compact Hausdorff space $X$ which tend towards zero at infinity (i.e. continuous functions $f$ on $X$ such that, for any $\epsilon > 0$, the set of points $x \in X$ which satisfy the condition $\abs{f(x)} \geq \epsilon$ is compact in $X$); $C_0(X)$ has the uniform norm
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| − | $$
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| − | \norm{f} = \sup_{x \in X} \abs{f(x)}.
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| − | $$
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| − | The involution in $C_0(X)$ is defined as transition to the complex-conjugate function: $f^*(x) = \overline{f(x)}$. Any commutative $C^*$-algebra $A$ is isometrically and symmetrically isomorphic (i.e. is isomorphic as a Banach algebra $A$ with involution) to the $C^*$-algebra $C_0(X)$, where $X$ is the space of maximal ideals of $A$ endowed with the Gel'fand topology
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| − | {{Cite|GeNe}},
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| − | {{Cite|Na}},
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| − | {{Cite|Di}}.
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| − | 2) The algebra $L(H)$ of all bounded linear operators on a Hilbert space $H$, considered with respect to the ordinary linear operations and operator multiplication. The involution in $L(H)$ is defined as transition to the adjoint operator, and the norm is defined as the ordinary operator norm.
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| − | A subset $M \subset A$ is said to be self-adjoint if $M = M^*$, where $M^* = \set{x^* : x \in M}$. Any closed self-adjoint subalgebra $B$ of a $C^*$-algebra $A$ is a $C^*$-algebra with respect to the linear operations, multiplication, involution, and norm taken from $A$; $B$ is said to be a $C^*$-subalgebra of $A$. Any $C^*$-algebra is isometrically and symmetrically isomorphic to a $C^*$-subalgebra of some $C^*$-algebra of the form $L(H)$. Any closed two-sided ideal $I$ in a $C^*$-algebra is self-adjoint (thus $I$ is a $C^*$-subalgebra of $A$), and the quotient algebra $A/I$, endowed with the natural linear operations, multiplication, involution, and quotient space norm, is a $C^*$-algebra. The set $K(H)$ of completely-continuous linear operators on a Hilbert space $H$ is a closed two-sided ideal in $L(H)$. If $A$ is a $C^*$-algebra and $\tilde{A}$ is the algebra with involution obtained from $A$ by addition of a unit element, there exists a unique norm on $\tilde{A} $ which converts $\tilde{A}$ into a $C^*$-algebra and which extends the norm on $A$. Moreover, the operations of bounded direct sum and tensor product
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| − | {{Cite|Di}},
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| − | {{Cite|Sa}} have been defined for $C^*$-algebras.
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| − | As in all symmetric Banach algebras with involution, in a $C^*$-algebra $A$ it is possible to define the following subsets: the real linear space $\Ah$ of Hermitian elements; the set of normal elements; the multiplicative group $U$ of unitary elements (if $A$ contains a unit element); and the set $A^+$ of positive elements. The set $A^+$ is a closed cone in $\Ah$, $A^+ \cap (-A)^+ = \set{0}$, $A^+ - A^+ = \Ah$, and the cone $A^+$ converts $\Ah$ into a real ordered vector space. If $A$ contains a unit element $1$, then $1$ is an interior point of the cone $A^+ \subset \Ah$. A linear functional $f$ on $A$ is called positive if $f(x) \geq 0 $ for all $x \in A^+$; such a functional is continuous. If $x \in B $, where $B$ is a $C^*$-subalgebra of $A$, the spectrum of $x$ in $B$ coincides with the spectrum of $x$ in $A$. The spectrum of a Hermitian element is real, the spectrum of a unitary element lies on the unit circle, and the spectrum of a positive element is non-negative. A functional calculus for the normal elements of a $C^*$-algebra has been constructed. Any $C^*$-algebra $A$ has an approximate unit, located in the unit ball of $A$ and formed by positive elements of $A$. If $I$, $J$ are closed two-sided ideals in $A$, then $(I+J)$ is a closed two-sided ideal in $A$ and $(I+J)^+ = I^+ + J^+$. If $I$ is a closed two-sided ideal in $J$ and $J$ is a closed two-sided ideal in $A$, then $I$ is a closed two-sided ideal in $A$. Any closed two-sided ideal is the intersection of the primitive two-sided ideals in which it is contained; any closed left ideal in $A$ is the intersection of the maximal regular left ideals in which it is contained.
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| − | Any $^*$-isomorphism of a $C^*$-algebra is isometric. Any $^*$-isomorphism $\pi$ of a Banach algebra $B$ with involution into a $C^*$-algebra $A$ is continuous, and $\norm{\pi(x)} \leq \norm{x}$ for all $x \in B$. In particular, all representations of a Banach algebra with involution (i.e. all $^*$-homomorphisms of $B$ into a $C^*$-algebra of the form $L(H)$) are continuous. The theory of representations of $C^*$-algebras forms a significant part of the theory of $C^*$-algebras, and the applications of the theory of $C^*$-algebras are related to the theory of representations of $C^*$-algebras. The properties of representations of $C^*$-algebras make it possible to construct for each $C^*$-algebra $A$ a topological space $\hat{A}$, called the spectrum of the $C^*$-algebra $A$, and to endow this space with a
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| − | [[Mackey–Borel structure|Mackey–Borel structure]]. In the general case, the spectrum of a $C^*$-algebra does not satisfy any separation axiom, but is a locally compact
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| − | [[Baire space|Baire space]].
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| − | A $C^*$-algebra $A$ is said to be a CCR-algebra (respectively, a GCR-algebra) if the relation $\pi(A) = K(H_\pi)$ (respectively, $\pi(A) \supset K(H_\pi)$) is satisfied for any non-null irreducible representation $\pi$ of the $C^*$-algebra $A$ in a Hilbert space $H$.
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| − | A $C^*$-algebra $A$ is said to be an NGCR-algebra if $A$ does not contain non-zero closed two-sided GCR-ideals (i.e. ideals which are GCR-algebras). Any $C^*$-algebra contains a maximal two-sided GCR-ideal $I$, and the quotient algebra $A/I$ is an NGCR-algebra. Any GCR-algebra contains an increasing family of closed two-sided ideals $I_\alpha$, indexed by ordinals $\alpha$, $\alpha \leq \rho $, such that $I_\rho = A$, $I_1=\set{0}$, $I_{\alpha+1}/I_\alpha$ is a CCR-algebra for all $\alpha < \rho$, and $I_\alpha = \union_{\alpha^\prime < \alpha} I_{\alpha^\prime}$ for limit ordinals $\alpha$. The spectrum of a GCR-algebra contains an open, everywhere-dense, separable, locally compact subset.
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| − | A $C^*$-algebra $ $ is said to be a $C^*$-algebra of type I if, for any representation $ $ of the $C^*$-algebra $ $ in a Hilbert space $ $, the
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| − | [[Von Neumann algebra|von Neumann algebra]] generated by the family $ $ in $ $ is a type I von Neumann algebra. For a $C^*$-algebra, the following conditions are equivalent: a) $ $ is a $C^*$-algebra of type I; b) $ $ is a $C^*$-algebra; and c) any quotient representation of the $C^*$-algebra $ $ is a multiple of the irreducible representation. If $ $ satisfies these conditions, then: 1) two irreducible representations of the $C^*$-algebra $ $ are equivalent if and only if their kernels are identical; and 2) the spectrum of the $C^*$-algebra $ $ is a $ $-space. If $ $ is a separable $C^*$-algebra, each of the conditions 1) and 2) is equivalent to the conditions a)–c). In particular, each separable $C^*$-algebra with a unique (up to equivalence) irreducible representation, is isomorphic to the $C^*$-algebra $ $ for some Hilbert space $ $.
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| − | Let $ $ be a $C^*$-algebra, and let $ $ be a set of elements $ $ such that the function $ $ is finite and continuous on the spectrum of $ $. If the linear envelope of $ $ is everywhere dense in $ $, then $ $ is said to be a $C^*$-algebra with continuous trace. The spectrum of such a $C^*$-algebra is separable and, under certain additional conditions, a $C^*$-algebra with a continuous trace may be represented as the algebra of vector functions on its spectrum $ $
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| − | {{Cite|Di}}.
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| − | Let $ $ be a $C^*$-algebra, let $ $ be the set of positive linear functionals on $ $ with norm $ $ and let $ $ be the set of non-zero boundary points of the convex set $ $. Then $ $ will be the set of pure states of $ $. Let $ $ be a $ $-subalgebra of $ $. If $ $ is a $C^*$-algebra and if $ $ separates the points of the set $ $, i.e. for any $ $, $ $, there exists an $ $ such that $ $, then $ $ (the Stone–Weierstrass theorem). If $ $ is any $C^*$-algebra and $ $ separates the points of the set $ $, then $ $.
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| − | The second dual space $ $ of a $C^*$-algebra $ $ is obviously provided with a multiplication converting $ $ into a $C^*$-algebra isomorphic to some von Neumann algebra; this algebra is named the von Neumann algebra enveloping the $C^*$-algebra
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| − | {{Cite|Di}},
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| − | {{Cite|Sa}}.
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| − | The theory of $C^*$-algebras has numerous applications in the theory of representations of groups and symmetric algebras
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| − | {{Cite|Di}}, the theory of dynamical systems
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| − | {{Cite|Sa}}, statistical physics and quantum field theory
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| − | {{Cite|Ru}}, and also in the theory of operators on a Hilbert space
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| − | {{Cite|Do}}.
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| − | ====Comments====
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| − | If $ $ over $ $ is an algebra with involution, i.e. if there is an operation $ $ satisfying $ $, $ $, $ $, the Hermitian, normal and positive elements are defined as follows. The element $ $ is a Hermitian element if $ $; it is a normal element if $ $ and it is a positive element if $ $ for some $ $. An element $ $ is a unitary element if $ $. An algebra with involution is also sometimes called a symmetric algebra (or symmetric ring), cf., e.g.,
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| − | {{Cite|Na}}. However, this usage conflicts with the concept of a symmetric algebra as a special kind of Frobenius algebra, cf.
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| − | [[Frobenius algebra|Frobenius algebra]].
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| − | Recent discoveries have revealed connections with, and applications to,
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| − | [[Algebraic topology|algebraic topology]]. If $ $ is a compact metrizable space, a group, $ $, can be formed from $ $-extensions of the compact operators by $ $,
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| − | $$ $$
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| − | In
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| − | {{Cite|BrDoFi}}, $ $ is shown to be a homotopy invariant functor of $ $ which may be identified with the topological $ $-homology group, $ $. In
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| − | {{Cite|At}} M.F. Atiyah attempted to make a description of $ $-homology, $ $, in terms of elliptic operators
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| − | {{Cite|Do2}}, p. 58. In
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| − | {{Cite|Ka}},
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| − | {{Cite|Ka2}} G.G. Kasparov developed a solution to this problem. Kasparov and others have used the equivariant version of Kasparov $ $-theory to prove the strong Novikov conjecture on higher signatures in many cases (see
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| − | {{Cite|Bl}}, pp. 309-314).
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| − | In addition, deep and novel connections between
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| − | [[K-theory|$ $-theory]] and operator algebras (cf.
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| − | [[Operator ring|Operator ring]]) were recently discovered by A. Connes
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| − | {{Cite|Co}}. Finally, V.F.R. Jones
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| − | {{Cite|Jo}} has exploited operator algebras to provide invariants of topological knots (cf.
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| − | [[Knot theory|Knot theory]]).
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| − | Further details on recent developments may be found in
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| − | {{Cite|Bl}},
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| − | {{Cite|Do2}}.
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| − | ====References====
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| − | {|
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| − | |valign="top"|{{Ref|At}}||valign="top"| M.F. Atiyah, "Global theory of elliptic operators", ''Proc. Internat. Conf. Funct. Anal. Related Topics'', Univ. Tokyo Press (1970) {{MR|0266247}} {{ZBL|0193.43601}}
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| − | |-
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| − | |valign="top"|{{Ref|Bl}}||valign="top"| B. Blackadar, "$ $-theory for operator algebras", Springer (1986) {{MR|0859867}} {{ZBL|0597.46072}}
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| − | |-
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| − | |valign="top"|{{Ref|BrDoFi}}||valign="top"| L.G. Brown, R.G. Douglas, P.A. Filmore, "Extensions of $C^*$-algebras and $ $-homology" ''Ann. of Math. (2)'', '''105''' (1977) pp. 265–324
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| − | |-
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| − | |valign="top"|{{Ref|Co}}||valign="top"| A. Connes, "Non-commutative differential geometry" ''Publ. Math. IHES'', '''62''' (1986) pp. 257–360 {{MR|}} {{ZBL|0657.55006}} {{ZBL|0592.46056}} {{ZBL|0564.58002}}
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| − | |-
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| − | |valign="top"|{{Ref|Di}}||valign="top"| J. Dixmier, "$ $ algebras", North-Holland (1977) (Translated from French) {{MR|0498740}} {{MR|0458185}} {{ZBL|0372.46058}} {{ZBL|0346.17010}} {{ZBL|0339.17007}}
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| − | |valign="top"|{{Ref|Do}}||valign="top"| R.G. Douglas, "Banach algebra techniques in operator theory", Acad. Press (1972) {{MR|0361893}} {{ZBL|0247.47001}}
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| − | |-
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| − | |valign="top"|{{Ref|Do2}}||valign="top"| R.G. Douglas, "$C^*$-algebra extensions and $ $-homology", Princeton Univ. Press (1980) {{MR|0571362}} {{ZBL|}}
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| − | |-
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| − | |valign="top"|{{Ref|GeNe}}||valign="top"| I.M. Gel'fand, M.A. [M.A. Naimark] Neumark, "On the imbedding of normed rings in the rings of operators in Hilbert space" ''Mat. Sb.'', '''12 (54)''' : 2 (1943) pp. 197–213 {{MR|9426}} {{ZBL|}}
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| − | |-
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| − | |valign="top"|{{Ref|Jo}}||valign="top"| V.F.R. Jones, "A polynomial invariant for knots via von Neumann algebras" ''Bull. Amer. Math. Soc.'', '''12''' (1985) pp. 103–111 {{MR|0766964}} {{ZBL|0564.57006}}
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| − | |-
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| − | |valign="top"|{{Ref|Ka}}||valign="top"| G.G. Kasparov, "The generalized index of elliptic operators" ''Funct. Anal. and Its Appl.'', '''7''' (1973) pp. 238–240 ''Funkt. Anal. i Prilozhen.'', '''7''' (1973) pp. 82–83 {{MR|445561}} {{ZBL|0305.58017}}
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| − | |-
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| − | |valign="top"|{{Ref|Ka2}}||valign="top"| G.G. Kasparov, "Topological invariants of elliptic operators I. $ $-homology" ''Math. USSR-Izv.'', '''9''' (1975) pp. 751–792 ''Izv. Akad. Nauk SSSR'', '''4''' (1975) pp. 796–838 {{MR|488027}} {{ZBL|}}
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| − | |-
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| − | |valign="top"|{{Ref|Na}}||valign="top"| M.A. Naimark, "Normed rings", Reidel (1984) (Translated from Russian) {{MR|1292007}} {{MR|0355601}} {{MR|0355602}} {{MR|0205093}} {{MR|0110956}} {{MR|0090786}} {{MR|0026763}} {{ZBL|0218.46042}} {{ZBL|0137.31703}} {{ZBL|0089.10102}} {{ZBL|0073.08902}}
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| − | |-
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| − | |valign="top"|{{Ref|Ru}}||valign="top"| D. Ruelle, "Statistical mechanics: rigorous results", Benjamin (1974) {{MR|0289084}} {{ZBL|0997.82506}} {{ZBL|1016.82500}} {{ZBL|0177.57301}}
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| − | |-
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| − | |valign="top"|{{Ref|Sa}}||valign="top"| S. Sakai, "$C^*$-algebras and $W^*$-algebras", Springer (1971) {{MR|0442701}} {{MR|0399878}} {{MR|0318902}} {{MR|0293415}} {{MR|0293414}} {{ZBL|}}
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| − | |-
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| − | |valign="top"|{{Ref|Ta}}||valign="top"| M. Takesaki, "Theory of operator algebras", '''1''', Springer (1979) {{MR|0548728}} {{ZBL|0436.46043}}
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