Banach algebra: Difference between revisions

From formulasearchengine
Jump to navigation Jump to search
en>AnomieBOT
m Dating maintenance tags: {{Clarify me}}
 
→‎Examples: linked C* algebra
Line 1: Line 1:
Do you see it difficult to obtain the greatest treatment for hemorrhoid? Then, we have come to the proper area considering this information may feature the principal causes of hemorrhoids with the appropriate hemorrhoid treatment that usually definitely end the bad experiences with this problem.<br><br>This is not a cure, nevertheless more of the treatment for the unpleasant symptoms of hemorrhoids. Aloe Vera Gel is availabe at most drugstores for inexpensive. After gently cleaning the region, apply a small amount of aloe vera to the affected area. This is repeated because required, nevertheless I found that twice a day was most effective for me. This natural [http://hemorrhoidtreatmentfix.com/thrombosed-hemorrhoid thrombosed hemorrhoid] is probably best for exterior hemorrhoids, or at least near-external.<br><br>As a previous hemorrhoid sufferer, I can tell you there are certain natural hemorrhoid treatments that you are able to do on your daily life which may help you or completely cure you when symptom is less serious. Try the following all-natural techniques before you look for alterantive treatment.<br><br>It is true that all these aspects cannot be covered by contemporary conventional treatments because they tend to work found on the symptoms alone plus not found on the cause of hemorrhoids. Hemorrhoids are caused due to numerous underlying wellness conditions which involve the digestive program, the belly, the bowel movements, the diet, the life-style and also the sleeping patterns. With conventional medicine, it is actually merely not possible to take care of all these aspects.<br><br>There are two types of hemorrhoids- internal and exterior. Both are the outcome of swollen veins in the anal area. Internal hemorrhoids is hard to discover since they are not noticeable. You'll just find out later when it starts to bleed. On the different hand, exterior hemorrhoids is felt because a difficult lump inside the anal opening. These are generally fairly noticeable because which they are swollen, red, itchy, plus very painful.<br><br>It comes inside 2 forms. One is a spray kind solution plus the different a natural supplement for the colon. Just spray Venapro below the language and instantly the magic of Venapro sets in. The elements trigger a natural immune inside response to the hemorrhoids. In brief, it "calms down" the symptoms of hemorrhoids.<br><br>When utilizing a sitz bath tub, you are able to use specialized soaps plus lotions that is produced to be employ with the bath. This may assist treat additional symptoms of the hemorrhoids, too. You can pick up any sitz tub at the regional health store or you can purchase one online.
In [[mathematics]], especially [[functional analysis]], a '''Banach algebra''', named after [[Stefan Banach]], is an [[associative algebra]] ''A'' over the [[real number|real]] or [[complex number|complex]] numbers which at the same time is also a [[Banach space]], i.e. normed and complete. The algebra multiplication and the Banach space norm are required to be related by the following inequality:
:<math> \forall x, y \in A : \|x \, y\| \ \leq  \|x \| \, \| y\| </math>
(i.e., the norm of the product is less than or equal to the product of the norms). This ensures that the multiplication operation is [[continuous function (topology)|continuous]]. This property is found in the real and complex numbers; for instance, |-6×5| ≤ |-6|×|5|.
 
If in the above we relax [[Banach space]] to [[normed space]] the analogous structure is called a '''normed algebra'''.
 
A Banach algebra is called "unital" if it has an [[identity element]] for the multiplication whose norm is 1, and "commutative" if its multiplication is [[commutative]].
Any Banach algebra <math>A</math> (whether it has an [[identity element]] or not) can be embedded isometrically into a unital Banach algebra <math>A_e</math> so as to form a closed ideal of <math>A_e</math>. Often one assumes ''a priori'' that the algebra under consideration is unital: for one can develop much of the theory by considering <math>A_e</math> and then applying the outcome in the original algebra. However, this is not the case all the time. For example, one cannot define all the trigonometric functions in a Banach algebra without identity.
 
The theory of real Banach algebras can be very different from the theory of complex Banach algebras. For example, the [[Spectrum of an operator|spectrum]] of an element of a nontrivial complex Banach algebra can never be empty, whereas in a real Banach algebra it could be empty for some elements.
 
Banach algebras can also be defined over fields of [[p-adic number]]s. This is part of [[p-adic analysis]].
 
== Examples ==
The prototypical example of a Banach algebra is <math>C_0(X)</math>, the space of (complex-valued) continuous functions on a locally compact (Hausdorff) space that vanish at infinity. <math>C_0(X)</math> is unital if and only if ''X'' is compact. The complex conjugation being an involution, <math>C_0(X)</math> is in fact a [[C*-algebra]]. More generally, every C*-algebra is a Banach algebra.
 
* The set of real (or complex) numbers is a Banach algebra with norm given by the [[absolute value]].
* The set of all real or complex ''n''-by-''n'' [[matrix (mathematics)|matrices]] becomes a [[unital algebra|unital]] Banach algebra if we equip it with a sub-multiplicative [[matrix norm]].
* Take the Banach space '''R'''<sup>''n''</sup>  (or '''C'''<sup>''n''</sup>) with norm ||''x''|| = max |''x''<sub>''i''</sub>| and define multiplication componentwise: (''x''<sub>1</sub>,...,''x''<sub>''n''</sub>)(''y''<sub>1</sub>,...,''y''<sub>''n''</sub>) = (''x''<sub>1</sub>''y''<sub>1</sub>,...,''x''<sub>''n''</sub>''y''<sub>''n''</sub>).
* The [[quaternion]]s form a 4-dimensional real Banach algebra, with the norm being given by the absolute value of quaternions.
* The algebra of all bounded real- or complex-valued functions defined on some set (with pointwise multiplication and the [[supremum]] norm) is a unital Banach algebra.
* The algebra of all bounded [[continuous function (topology)|continuous]] real- or complex-valued functions on some [[locally compact space]] (again with pointwise operations and supremum norm) is a Banach algebra.
* The algebra of all [[continuous function (topology)|continuous]] [[linear transformation|linear]] operators on a Banach space E (with functional composition as multiplication and the [[operator norm]] as norm) is a unital Banach algebra. The set of all [[compact operator]]s on E is a closed ideal in this algebra.
* If ''G'' is a [[locally compact]] [[Hausdorff space|Hausdorff]] [[topological group]] and μ its [[Haar measure]], then the Banach space L<sup>1</sup>(''G'') of all μ-integrable functions on ''G'' becomes a Banach algebra under the [[convolution]] ''xy''(''g'') = ∫ ''x''(''h'') ''y''(''h''<sup>−1</sup>''g'') dμ(''h'') for ''x'', ''y'' in L<sup>1</sup>(''G'').
* [[Uniform algebra]]: A Banach algebra that is a subalgebra of C(X) with the supremum norm and that contains the constants and separates the points of X (which must be a compact Hausdorff space).
* [[Uniform algebra|Natural Banach function algebra]]: A uniform algebra whose all characters are evaluations at points of X.
* [[C*-algebra]]: A Banach algebra that is a closed *-subalgebra of the algebra of bounded operators on some [[Hilbert space]].
* [[Measure algebra]]: A Banach algebra consisting of all [[Radon measure]]s on some [[locally compact group]], where the product of two measures is given by [[convolution]].
 
== Properties ==
Several [[list of functions|elementary functions]] which are defined via [[power series]] may be defined in any unital Banach algebra; examples include the [[exponential function]] and the [[trigonometric function]]s, and more generally any [[entire function]]. (In particular, the exponential map can be used to define [[abstract index group]]s.) The formula for the [[geometric series]] remains valid in general unital Banach algebras. The [[binomial theorem]] also holds for two commuting elements of a Banach algebra.
 
The set of [[invertible element]]s in any unital Banach algebra is an [[open set]], and the inversion operation on this set is continuous, (and hence homeomorphism) so that it forms a [[topological group]] under multiplication.
 
If a Banach algebra has unit '''1''', then '''1''' cannot be a [[Commutator#Ring_theory|commutator]]; i.e., <math>xy - yx \ne \mathbf{1}</math>&thinsp; for any ''x'', ''y''&nbsp;∈&nbsp;''A''.
 
The various algebras of functions given in the examples above have very different properties from standard examples of algebras such as the reals. For example:
 
* Every real Banach algebra which is a [[division algebra]] is isomorphic to the reals, the complexes, or the quaternions. Hence, the only complex Banach algebra which is a division algebra is the complexes. (This is known as the [[Gelfand-Mazur theorem]].)
 
* Every unital real Banach algebra with no [[zero divisor]]s, and in which every [[principal ideal]] is [[closed set|closed]], is isomorphic to the reals, the complexes, or the quaternions.
 
* Every commutative real unital [[Noetherian ring|Noetherian]] Banach algebra with no zero divisors is isomorphic to the real or complex numbers.
 
* Every commutative real unital Noetherian Banach algebra (possibly having zero divisors) is finite-dimensional.
 
* Permanently singular elements in Banach algebras are [[topological divisior of zero|topological divisors of zero]], ''i.e.'', considering extensions ''B'' of Banach algebras ''A'' some elements that are singular in the given algebra ''A'' have a multiplicative inverse element in a Banach algebra extension ''B''.  Topological divisors of zero in ''A'' are permanently singular in all Banach extension ''B'' of&nbsp;''A''.
 
== Spectral theory ==
{{main|Spectral theory}}
 
Unital Banach algebras over the complex field provide a general setting to develop spectral theory. The ''spectrum'' of an element ''x''&nbsp;∈&nbsp;''A'', denoted by <math>\sigma(x)</math>, consists of all those complex [[scalar (mathematics)|scalar]]s ''λ'' such that ''x''&nbsp;&minus;&nbsp;''λ'''''1''' is not invertible in ''A''.  The spectrum of any element ''x'' is a closed subset of the closed disc in '''C''' with radius ||''x''|| and center 0, and thus is  [[Compact space|compact]]. Moreover, the spectrum <math>\sigma(x)</math> of an element ''x'' is [[non-empty]] and satisfies the [[spectral radius]] formula:
 
:<math>\sup \{ |\lambda| : \lambda \in \sigma(x) \} = \lim_{n \to \infty} \|x^n\|^{1/n}.</math>
 
Given ''x''&nbsp;&isin;&nbsp;''A'', the [[holomorphic functional calculus]] allows to define ''ƒ''(''x'')&nbsp;∈ ''A'' for any function ''ƒ'' [[holomorphic function|holomorphic]] in a neighborhood of <math>\sigma(x).</math>  Furthermore, the spectral mapping theorem holds:
 
:<math>\sigma(f(x)) = f(\sigma(x)).</math><ref>Takesaki, Theory of Operator Algebras I. Proposition 2.8.</ref>
 
When the Banach algebra ''A'' is the algebra L(''X'') of bounded linear operators on a complex Banach space ''X''&thinsp; (e.g., the algebra of square matrices), the notion of the spectrum in ''A'' coincides with the usual one in the operator theory. For ''ƒ''&nbsp;&isin; ''C''(''X'') (with a compact Hausdorff space&nbsp;''X''), one sees that:
 
:<math>\sigma(f) = \{ f(t) : t \in X \}.</math>
 
The norm of a normal element ''x'' of a C*-algebra coincides with its spectral radius.  This generalizes an analogous fact for normal operators.
 
Let ''A''&thinsp; be a complex unital Banach algebra in which every non-zero element ''x'' is invertible (a division algebra). For every ''a''&nbsp;&isin; ''A'', there is ''λ''&nbsp;&isin; '''C''' such that
''a''&nbsp;&minus;&nbsp;''λ'''''1''' is not invertible (because the spectrum of ''a'' is not empty) hence ''a''&nbsp;=&nbsp;''λ'''''1'''&nbsp;:  this algebra ''A'' is naturally isomorphic to '''C''' (the complex case of the Gelfand-Mazur theorem).
 
== Ideals and characters ==
Let ''A''&thinsp; be a unital ''commutative'' Banach algebra over '''C'''. Since ''A'' is then a commutative ring with unit, every non-invertible element of ''A'' belongs to some [[maximal ideal]] of ''A''. Since a maximal ideal <math>\mathfrak m</math> in ''A'' is closed, <math>A / \mathfrak m</math> is a Banach algebra that is a field, and it follows from the Gelfand-Mazur theorem that there is a bijection between the set of all maximal ideals of ''A'' and the set Δ(''A'') of all nonzero homomorphisms from ''A''&thinsp; to '''C'''. The set Δ(''A'') is called the "[[structure space]]" or "character space" of ''A'', and its members "characters."
 
A character χ is a linear functional on ''A'' which is at the same time multiplicative, χ(''ab'') = χ(''a'') χ(''b''), and satisfies ''χ''('''1''') = 1.  Every character is automatically continuous from ''A''&thinsp; to '''C''', since the kernel of a character is a maximal ideal, which is closed. Moreover, the norm (''i.e.'', operator norm) of a character is one. Equipped with the topology of pointwise convergence on ''A'' (''i.e.'', the topology induced by the weak-* topology of&nbsp;''A''<sup></sup>), the character space, Δ(''A''), is a Hausdorff compact space.
 
For any ''x'' ∈ ''A'',
 
:<math>\sigma(x) = \sigma(\hat x)</math>
 
where <math>\hat x</math> is the [[Gelfand representation]] of ''x'' defined as follows: <math>\hat x</math> is the continuous function from Δ(''A'') to '''C''' given by <math>\hat x(\chi) = \chi(x).</math>&thinsp;  The spectrum of <math>\hat x,</math> in the formula above, is the spectrum as element of the algebra ''C''(Δ(''A'')) of complex continuous functions on the compact space Δ(''A'').  Explicitly,
 
:<math>\sigma(\hat x) = \{ \chi(x) : \chi \in \Delta(A) \}</math>.
 
As an algebra, a unital commutative Banach algebra is [[semisimple algebra|semisimple]] (i.e., its [[Jacobson radical]] is zero) if and only if its Gelfand representation has trivial kernel. An important example of such an algebra is a commutative C*-algebra. In fact, when ''A'' is a commutative unital C*-algebra, the Gelfand representation is then an isometric *-isomorphism between ''A'' and  ''C''(Δ(''A'')) .<ref>Proof: Since every element of a commutative C*-algebra is normal, the Gelfand representation is isometric; in particular, it is injective and its image is closed. But the image of the Gelfand representation is dense by the [[Stone-Weierstrass theorem]].</ref>
 
== See also ==
* [[Operator algebras]]
* [[Shilov boundary]]
* [[Automatic continuity]]
* [[Kaplansky's conjecture]]
* [[Approximate identity]]
 
==Notes==
{{Reflist}}
 
==References==
* {{cite book | author=Béla Bollobás | authorlink=Béla Bollobás | title=Linear Analysis | publisher=Cambridge University Press | year=1990 | isbn=0-521-38729-9 }}
* {{cite book | author=Frank F. Bonsall, John Duncan | title=Complete Normed Algebras | publisher=Springer-Verlag, New York | year=1973 | isbn=0-387-06386-2}}
* {{cite book | author=H. Garth Dales, Pietro Aeina, Jörg Eschmeier, Kjeld Laursen, George A. Willis | title=Introduction to Banach Algebras, Operators and Harmonic Analysis | series=Cambridge University Press | year=2003 | isbn=0-521-53584-0 }}
* {{cite book | author=Richard D. Mosak | title=Banach algebras | series=Chicago Lectures in Mathematics | year=1975 | isbn=0-226-54203-3 }}
 
{{DEFAULTSORT:Banach Algebra}}
[[Category:Banach algebras| ]]
[[Category:Fourier analysis]]
[[Category:Science and technology in Poland]]

Revision as of 22:44, 17 November 2013

In mathematics, especially functional analysis, a Banach algebra, named after Stefan Banach, is an associative algebra A over the real or complex numbers which at the same time is also a Banach space, i.e. normed and complete. The algebra multiplication and the Banach space norm are required to be related by the following inequality:

(i.e., the norm of the product is less than or equal to the product of the norms). This ensures that the multiplication operation is continuous. This property is found in the real and complex numbers; for instance, |-6×5| ≤ |-6|×|5|.

If in the above we relax Banach space to normed space the analogous structure is called a normed algebra.

A Banach algebra is called "unital" if it has an identity element for the multiplication whose norm is 1, and "commutative" if its multiplication is commutative. Any Banach algebra (whether it has an identity element or not) can be embedded isometrically into a unital Banach algebra so as to form a closed ideal of . Often one assumes a priori that the algebra under consideration is unital: for one can develop much of the theory by considering and then applying the outcome in the original algebra. However, this is not the case all the time. For example, one cannot define all the trigonometric functions in a Banach algebra without identity.

The theory of real Banach algebras can be very different from the theory of complex Banach algebras. For example, the spectrum of an element of a nontrivial complex Banach algebra can never be empty, whereas in a real Banach algebra it could be empty for some elements.

Banach algebras can also be defined over fields of p-adic numbers. This is part of p-adic analysis.

Examples

The prototypical example of a Banach algebra is , the space of (complex-valued) continuous functions on a locally compact (Hausdorff) space that vanish at infinity. is unital if and only if X is compact. The complex conjugation being an involution, is in fact a C*-algebra. More generally, every C*-algebra is a Banach algebra.

  • The set of real (or complex) numbers is a Banach algebra with norm given by the absolute value.
  • The set of all real or complex n-by-n matrices becomes a unital Banach algebra if we equip it with a sub-multiplicative matrix norm.
  • Take the Banach space Rn (or Cn) with norm ||x|| = max |xi| and define multiplication componentwise: (x1,...,xn)(y1,...,yn) = (x1y1,...,xnyn).
  • The quaternions form a 4-dimensional real Banach algebra, with the norm being given by the absolute value of quaternions.
  • The algebra of all bounded real- or complex-valued functions defined on some set (with pointwise multiplication and the supremum norm) is a unital Banach algebra.
  • The algebra of all bounded continuous real- or complex-valued functions on some locally compact space (again with pointwise operations and supremum norm) is a Banach algebra.
  • The algebra of all continuous linear operators on a Banach space E (with functional composition as multiplication and the operator norm as norm) is a unital Banach algebra. The set of all compact operators on E is a closed ideal in this algebra.
  • If G is a locally compact Hausdorff topological group and μ its Haar measure, then the Banach space L1(G) of all μ-integrable functions on G becomes a Banach algebra under the convolution xy(g) = ∫ x(h) y(h−1g) dμ(h) for x, y in L1(G).
  • Uniform algebra: A Banach algebra that is a subalgebra of C(X) with the supremum norm and that contains the constants and separates the points of X (which must be a compact Hausdorff space).
  • Natural Banach function algebra: A uniform algebra whose all characters are evaluations at points of X.
  • C*-algebra: A Banach algebra that is a closed *-subalgebra of the algebra of bounded operators on some Hilbert space.
  • Measure algebra: A Banach algebra consisting of all Radon measures on some locally compact group, where the product of two measures is given by convolution.

Properties

Several elementary functions which are defined via power series may be defined in any unital Banach algebra; examples include the exponential function and the trigonometric functions, and more generally any entire function. (In particular, the exponential map can be used to define abstract index groups.) The formula for the geometric series remains valid in general unital Banach algebras. The binomial theorem also holds for two commuting elements of a Banach algebra.

The set of invertible elements in any unital Banach algebra is an open set, and the inversion operation on this set is continuous, (and hence homeomorphism) so that it forms a topological group under multiplication.

If a Banach algebra has unit 1, then 1 cannot be a commutator; i.e.,   for any x, y ∈ A.

The various algebras of functions given in the examples above have very different properties from standard examples of algebras such as the reals. For example:

  • Every real Banach algebra which is a division algebra is isomorphic to the reals, the complexes, or the quaternions. Hence, the only complex Banach algebra which is a division algebra is the complexes. (This is known as the Gelfand-Mazur theorem.)
  • Every commutative real unital Noetherian Banach algebra with no zero divisors is isomorphic to the real or complex numbers.
  • Every commutative real unital Noetherian Banach algebra (possibly having zero divisors) is finite-dimensional.
  • Permanently singular elements in Banach algebras are topological divisors of zero, i.e., considering extensions B of Banach algebras A some elements that are singular in the given algebra A have a multiplicative inverse element in a Banach algebra extension B. Topological divisors of zero in A are permanently singular in all Banach extension B of A.

Spectral theory

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church.

Unital Banach algebras over the complex field provide a general setting to develop spectral theory. The spectrum of an element x ∈ A, denoted by , consists of all those complex scalars λ such that x − λ1 is not invertible in A. The spectrum of any element x is a closed subset of the closed disc in C with radius ||x|| and center 0, and thus is compact. Moreover, the spectrum of an element x is non-empty and satisfies the spectral radius formula:

Given x ∈ A, the holomorphic functional calculus allows to define ƒ(x) ∈ A for any function ƒ holomorphic in a neighborhood of Furthermore, the spectral mapping theorem holds:

[1]

When the Banach algebra A is the algebra L(X) of bounded linear operators on a complex Banach space X  (e.g., the algebra of square matrices), the notion of the spectrum in A coincides with the usual one in the operator theory. For ƒ ∈ C(X) (with a compact Hausdorff space X), one sees that:

The norm of a normal element x of a C*-algebra coincides with its spectral radius. This generalizes an analogous fact for normal operators.

Let A  be a complex unital Banach algebra in which every non-zero element x is invertible (a division algebra). For every a ∈ A, there is λ ∈ C such that a − λ1 is not invertible (because the spectrum of a is not empty) hence a = λ1 : this algebra A is naturally isomorphic to C (the complex case of the Gelfand-Mazur theorem).

Ideals and characters

Let A  be a unital commutative Banach algebra over C. Since A is then a commutative ring with unit, every non-invertible element of A belongs to some maximal ideal of A. Since a maximal ideal in A is closed, is a Banach algebra that is a field, and it follows from the Gelfand-Mazur theorem that there is a bijection between the set of all maximal ideals of A and the set Δ(A) of all nonzero homomorphisms from A  to C. The set Δ(A) is called the "structure space" or "character space" of A, and its members "characters."

A character χ is a linear functional on A which is at the same time multiplicative, χ(ab) = χ(a) χ(b), and satisfies χ(1) = 1. Every character is automatically continuous from A  to C, since the kernel of a character is a maximal ideal, which is closed. Moreover, the norm (i.e., operator norm) of a character is one. Equipped with the topology of pointwise convergence on A (i.e., the topology induced by the weak-* topology of A), the character space, Δ(A), is a Hausdorff compact space.

For any xA,

where is the Gelfand representation of x defined as follows: is the continuous function from Δ(A) to C given by   The spectrum of in the formula above, is the spectrum as element of the algebra C(Δ(A)) of complex continuous functions on the compact space Δ(A). Explicitly,

.

As an algebra, a unital commutative Banach algebra is semisimple (i.e., its Jacobson radical is zero) if and only if its Gelfand representation has trivial kernel. An important example of such an algebra is a commutative C*-algebra. In fact, when A is a commutative unital C*-algebra, the Gelfand representation is then an isometric *-isomorphism between A and C(Δ(A)) .[2]

See also

Notes

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

References

  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  1. Takesaki, Theory of Operator Algebras I. Proposition 2.8.
  2. Proof: Since every element of a commutative C*-algebra is normal, the Gelfand representation is isometric; in particular, it is injective and its image is closed. But the image of the Gelfand representation is dense by the Stone-Weierstrass theorem.