# weak Hopf C*-algebra

###### Definition 0.1.

A weak Hopf $C^{*}$-algebra is defined as a weak Hopf algebra (http://planetmath.org/WeakHopfCAlgebra) which admits a faithful $*$–representation on a Hilbert space. The weak C*–Hopf algebra is therefore much more likely to be closely related to a quantum groupoid than the weak Hopf algebra. However, one can argue that locally compact groupoids equipped with a Haar measure are even closer to defining quantum groupoids (http://planetmath.org/QuantumGroupoids2).

There are already several, significant examples that motivate the consideration of weak C*-Hopf algebras which also deserve mentioning in the context of standard quantum theories. Furthermore, notions such as (proper) weak C*-algebroids can provide the main framework for symmetry breaking and quantum gravity that we are considering here. Thus, one may consider the quasi-group symmetries constructed by means of special transformations of the coordinate space $M$.

Remark: Recall that the weak Hopf algebra is defined as the extension of a Hopf algebra by weakening the definining axioms of a Hopf algebra as follows:

• (1)

The comultiplication is not necessarily unit-preserving.

• (2)

The counit $\varepsilon$ is not necessarily a homomorphism of algebras.

• (3)

The axioms for the antipode map $S:A{\longrightarrow}A$ with respect to the counit are as follows. For all $h\in H$,

 $\displaystyle m({\rm id}\otimes S)\Delta(h)$ $\displaystyle=(\varepsilon\otimes{\rm id})(\Delta(1)(h\otimes 1))$ (0.1) $\displaystyle m(S\otimes{\rm id})\Delta(h)$ $\displaystyle=({\rm id}\otimes\varepsilon)((1\otimes h)\Delta(1))$ $\displaystyle S(h)$ $\displaystyle=S(h_{(1)})h_{(2)}S(h_{(3)})~{}.$

These axioms may be appended by the following commutative diagrams

 ${\begin{CD}A\otimes A@>{S\otimes{\rm id}}>{}>A\otimes A\\ @A{\Delta}A{}A@V{}V{m}V\\ A@ >u\circ\varepsilon>>A\end{CD}}\qquad{\begin{CD}A\otimes A@>{{\rm id}\otimes S% }>{}>A\otimes A\\ @A{\Delta}A{}A@V{}V{m}V\\ A@ >u\circ\varepsilon>>A\end{CD}}$ (0.2)

along with the counit axiom:

 $\xymatrix@C=3pc@R=3pc{A\otimes A\ar[d]_{\varepsilon\otimes 1}&A\ar[l]_{\Delta}% \ar[dl]_{{\rm id}_{A}}\ar[d]^{\Delta}\\ A&A\otimes A\ar[l]^{1\otimes\varepsilon}}$ (0.3)

Some authors substitute the term quantum groupoid for a weak Hopf algebra.

## 0.1 Examples of weak Hopf C*-algebra.

• (1)

In Nikshych and Vainerman (2000) quantum groupoids were considered as weak C*–Hopf algebras and were studied in relationship to the noncommutative symmetries of depth 2 von Neumann subfactors. If

 $A\subset B\subset B_{1}\subset B_{2}\subset\ldots$ (0.4)

is the Jones extension induced by a finite index depth $2$ inclusion $A\subset B$ of $II_{1}$ factors, then $Q=A^{\prime}\cap B_{2}$ admits a quantum groupoid structure and acts on $B_{1}$, so that $B=B_{1}^{Q}$ and $B_{2}=B_{1}\rtimes Q$ . Similarly, in Rehren (1997) ‘paragroups’ (derived from weak C*–Hopf algebras) comprise (quantum) groupoids of equivalence classes such as associated with 6j–symmetry groups (relative to a fusion rules algebra). They correspond to type $II$ von Neumann algebras in quantum mechanics, and arise as symmetries where the local subfactors (in the sense of containment of observables within fields) have depth $2$ in the Jones extension. Related is how a von Neumann algebra $N$, such as of finite index depth $2$, sits inside a weak Hopf algebra formed as the crossed product $N\rtimes A$ (Böhm et al. 1999).

• (2)

In Mack and Schomerus (1992) using a more general notion of the Drinfeld construction, develop the notion of a quasi triangular quasi–Hopf algebra (QTQHA) is developed with the aim of studying a range of essential symmetries with special properties, such the quantum group algebra ${\rm U}_{q}(\rm{sl}_{2})$ with $|q|=1$ . If $q^{p}=1$, then it is shown that a QTQHA is canonically associated with ${\rm U}_{q}(\rm{sl}_{2})$. Such QTQHAs are claimed as the true symmetries of minimal conformal field theories.

## 0.2 Von Neumann Algebras (or $W^{*}$-algebras).

Let $\mathcal{H}$ denote a complex (separable) Hilbert space. A von Neumann algebra $\mathcal{A}$ acting on $\mathcal{H}$ is a subset of the $*$–algebra of all bounded operators $\mathcal{L}(\mathcal{H})$ such that:

• (1)

$\mathcal{A}$ is closed under the adjoint operation (with the adjoint of an element $T$ denoted by $T^{*}$).

• (2)

$\mathcal{A}$ equals its bicommutant, namely:

 $\mathcal{A}=\{A\in\mathcal{L}(\mathcal{H}):\forall B\in\mathcal{L}(\mathcal{H}% ),\forall C\in\mathcal{A},~{}(BC=CB)\Rightarrow(AB=BA)\}~{}.$ (0.5)

If one calls a commutant of a set $\mathcal{A}$ the special set of bounded operators on $\mathcal{L}(\mathcal{H})$ which commute with all elements in $\mathcal{A}$, then this second condition implies that the commutant of the commutant of $\mathcal{A}$ is again the set $\mathcal{A}$.

On the other hand, a von Neumann algebra $\mathcal{A}$ inherits a unital subalgebra from $\mathcal{L}(\mathcal{H})$, and according to the first condition in its definition $\mathcal{A}$ does indeed inherit a *-subalgebra structure, as further explained in the next section on C*-algebras. Furthermore, we have the notable Bicommutant Theorem which states that $\mathcal{A}$ is a von Neumann algebra if and only if $\mathcal{A}$ is a *-subalgebra of $\mathcal{L}(\mathcal{H})$, closed for the smallest topology defined by continuous maps $(\xi,\eta)\longmapsto(A\xi,\eta)$ for all $$ where $<.,.>$ denotes the inner product defined on $\mathcal{H}$ . For further instruction on this subject, see e.g. Aflsen and Schultz (2003), Connes (1994).

Commutative and noncommutative Hopf algebras form the backbone of quantum ‘groups’ and are essential to the generalizations of symmetry. Indeed, in most respects a quantum ‘group’ is identifiable with a Hopf algebra. When such algebras are actually associated with proper groups of matrices there is considerable scope for their representations on both finite and infinite dimensional Hilbert spaces.

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• 25 http://planetmath.org/?op=getobj&from=books&id=294Stefaan Vaes and Leonid Vainerman.2003. On Low-Dimensional Locally Compact Quantum Groups in Locally Compact Quantum Groups and Groupoids: Proceedings of the Meeting of Theoretical Physicists and Mathematicians
 Title weak Hopf C*-algebra Canonical name WeakHopfCalgebra Date of creation 2013-03-22 18:12:47 Last modified on 2013-03-22 18:12:47 Owner bci1 (20947) Last modified by bci1 (20947) Numerical id 62 Author bci1 (20947) Entry type Topic Classification msc 08C99 Classification msc 16S40 Classification msc 81R15 Classification msc 81R50 Classification msc 16W30 Classification msc 57T05 Synonym quantum groupoids Related topic WeakHopfAlgebra Related topic VonNeumannAlgebra Related topic TopologicalAlgebra Related topic QuantumGroupoids2 Related topic LocallyCompactQuantumGroup Related topic HopfAlgebra Related topic LocallyCompactGroupoids Related topic QuantumGroupoids2 Related topic GroupoidAndGroupRepresentationsRelatedToQuantumSymmetries Related topic GrassmanHopfAlgebrasAndTheirDualCoAlge Defines weak Hopf algebra Defines weak Hopf C*-algebra Defines weak bialgebra Defines quantum group Defines quantum groupoid Defines von Neumann algebra Defines $W^{*}$–algebra