# PlanetPhysics/Jordan Banach and Jordan Lie Algebras

\subsubsection{Jordan-Banach, Jordan-Lie, and Jordan-Banach-Lie algebras: Definitions and Relationships to Poisson and C*-algebras}

Firstly, a specific *algebra* consists of a vector space over a ground field (typically **Failed to parse (unknown function "\bR"): {\displaystyle \bR}**
or **Failed to parse (unknown function "\bC"): {\displaystyle \bC}**
) equipped with a bilinear and distributive multiplication ~. Note that is not necessarily commutative or associative.

A *Jordan algebra* (over **Failed to parse (unknown function "\bR"): {\displaystyle \bR}**
), is an algebra over **Failed to parse (unknown function "\bR"): {\displaystyle \bR}**
for which:

</math> S \circ T &= T \circ S~, \\ S \circ (T \circ S^2) &= (S \circ T) \circ S^2 of the algebra.

It is worthwhile noting now that in the algebraic theory of Jordan algebras, an important role is played by the *Jordan triple product* as defined by:

which is linear in each factor and for which ~. Certain examples entail setting ~.

A *Jordan Lie Algebra* is a real vector space **Failed to parse (unknown function "\bR"): {\displaystyle \mathfrak A_{\bR}}**
together with a *Jordan product* and *Poisson bracket*

, satisfying~:

\item[1.] for allFailed to parse (unknown function "\bR"): {\displaystyle S, T \in \mathfrak A_{\bR}}, </math> S \circ T &= T \circ S \\ \{S, T \} &= - \{T, S\}Failed to parse (unknown function "\item"): {\displaystyle \item[2.] the ''Leibniz rule'' holds <math> \{S, T \circ W \} = \{S, T\} \circ W + T \circ \{S, W\},}for allFailed to parse (unknown function "\bR"): {\displaystyle S, T, W \in \mathfrak A_{\bR}}, along with \item[3.] theJacobi identity~: \item[4.] for someFailed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "/mathoid/local/v1/":): {\displaystyle \hslash^2 \in \bR}, there is theassociator identity~:

#### Poisson algebraEdit

By a *Poisson algebra* we mean a Jordan algebra in which is associative. The
usual algebraic types of morphisms automorphism, isomorphism, etc.) apply to Jordan-Lie (Poisson) algebras (see Landsman, 2003).

Consider the classical configuration space **Failed to parse (unknown function "\bR"): {\displaystyle Q = \bR^3}**
of a moving particle whose phase space is the cotangent bundle **Failed to parse (unknown function "\bR"): {\displaystyle T^* \bR^3 \cong \bR^6}**
, and for which the space of (classical) observables is taken to be the real vector space of smooth functions **Failed to parse (unknown function "\bR"): {\displaystyle \mathfrak A^0_{\bR} = C^{\infty}(T^* R^3, \bR)}**
~. The usual pointwise multiplication of functions defines a bilinear map on **Failed to parse (unknown function "\bR"): {\displaystyle \mathfrak A^0_{\bR}}**
, which is seen to be commutative and associative. Further, the Poisson bracket on functions

**Failed to parse (unknown function "\del"): {\displaystyle \{f, g \} := \frac{\del f}{\del p^i} \frac{\del g}{\del q_i} - \frac{\del f}{\del q_i} \frac{\del g}{\del p^i} ~,}**

which can be easily seen to satisfy the Liebniz rule above. The axioms above then set the stage of passage to quantum mechanical systems which the parameter suggests.

#### C*--algebras (C*--A), JLB and JBW AlgebrasEdit

An *involution* on a complex algebra is a real--linear map such that for all and
**Failed to parse (unknown function "\bC"): {\displaystyle \lambda \in \bC}**
, we have also

A **--algebra* is said to be a complex associative algebra together with an involution ~.

A *C*--algebra* is a simultaneously a *--algebra and a Banach space , satisfying for all ~:
\bigbreak

</math> \Vert S \circ T \Vert &\leq \Vert S \Vert ~ \Vert T \Vert~, \\ \Vert T^* T \Vert^2 & = \Vert T\Vert^2 ~. ~. By the above axioms a C*--algebra is a special case of a Banach algebra where the latter requires the above norm property but not the involution (*) property. Given Banach spaces the space of (bounded) linear operators from to forms a Banach space, where for , the space is a Banach algebra with respect to the norm

In quantum field theory one may start with a Hilbert space , and consider the Banach algebra of bounded linear operators which given to be closed under the usual algebraic operations and taking adjoints, forms a --algebra of bounded operators, where the adjoint operation functions as the involution, and for we have~:

and </math> \Vert Tu \Vert^2 = (Tu,
Tu) = (u, T^*Tu) \leq \Vert T^* T \Vert~ \Vert u \Vert^2~. we mean a linear map **Failed to parse (unknown function "\lra"): {\displaystyle \phi : \mathfrak A \lra \mathfrak B}**
, such that for all , the following hold~:

where a bijective morphism is said to be an isomorphism (in which case it is then an isometry). A fundamental relation is that any norm-closed --algebra in is a C*--algebra, and conversely, any C*--algebra is isomorphic to a norm--closed --algebra in for some Hilbert space ~.

For a C*--algebra , we say that is *self--adjoint* if ~. Accordingly, the self--adjoint part of is a real vector space since we can decompose as ~:

A *commutative* C*--algebra is one for which the associative multiplication is commutative. Given a commutative C*--algebra , we have , the algebra of continuous functions on a compact Hausdorff space .

A *Jordan--Banach algebra* (a JB--algebra for short) is both a real Jordan algebra and a Banach space, where for all **Failed to parse (unknown function "\bR"): {\displaystyle S, T \in \mathfrak A_{\bR}}**
, we have

**Failed to parse (syntax error): {\displaystyle \Vert S \circ T \Vert &\leq \Vert S \Vert ~ \Vert T \Vert ~, \\ \Vert T \Vert^2 &\leq \Vert S^2 + T^2 \Vert ~. }**

A *JLB--algebra* is a JB--algebra **Failed to parse (unknown function "\bR"): {\displaystyle \mathfrak A_{\bR}}**
together with a Poisson bracket for which it becomes a Jordan--Lie algebra for some ~. Such JLB--algebras often constitute the real part of several widely studied complex associative algebras.

For the purpose of quantization, there are fundamental relations between , JLB and Poisson algebras.

Conversely, given a JLB--algebra **Failed to parse (unknown function "\bR"): {\displaystyle \mathfrak A_{\bR}}**
with , its
complexification is a -algebra under the operations~:

**Failed to parse (syntax error): {\displaystyle S T &:= S \circ T - \frac{\iota}{2} k \times{\left\{S,T\right\}}_k ~, {(S + \iota T)}^* &:= S-\iota T . }**

For further details see Landsman (2003) (Thm. 1.1.9).

A JB--algebra which is monotone complete and admits a separating set of normal sets is called a *JBW-algebra*. These appeared in the work of von Neumann who developed a (orthomodular) lattice theory of projections on on which to study quantum logic. BW-algebras have the following property: whereas is a J(L)B--algebra, the self adjoint part of a von Neumann algebra is a JBW--algebra.

A *JC--algebra* is a norm closed real linear subspace of </math>\mathcal
L(H)^{sa}S \circ T = \frac{1}{2}(ST + TS)\mathcal L(H)^{sa}**Failed to parse (syntax error): {\displaystyle is a JB--algebra, it is natural to specify the exact relationship between JB and JC--algebras, at least in finite dimensions. In order to do this, one introduces the `exceptional' algebra }**
H_3({\mathbb O})3 \times 3\mathbb O as a (direct) summand ^{[1]}.

The above definitions and constructions follow the approach of Alfsen and Schultz (2003), and also reported earlier by Landsman (1998).

## All SourcesEdit

^{[1]}

## ReferencesEdit

- ↑
^{1.0}^{1.1}Alfsen, E.M. and F. W. Schultz: Geometry of State Spaces of Operator Algebras, Birkh\"auser, Boston-Basel-Berlin.(2003).