# PlanetPhysics/Compact Quantum Groupoids

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### Introduction and basic conceptsEdit

Compact quantum groupoids were introduced in Landsman (1998) as a
simultaneous generalization of a compact groupoid and a quantum group. Since this construction is relevant to the definition of locally compact quantum groupoids and their representations investigated here, its exposition is required before we can step up to the next level of generality. Firstly, let and denote C*--algebras equipped with a *--homomorphism **Failed to parse (unknown function "\lra"): {\displaystyle \eta_s : \mathfrak B \lra \mathfrak A}**
, and a *--antihomomorphism **Failed to parse (unknown function "\lra"): {\displaystyle \eta_t : \mathfrak B \lra \mathfrak A}**
whose images in
commute. A non--commutative Haar measure is defined as a completely
positive map **Failed to parse (unknown function "\lra"): {\displaystyle P: \mathfrak A \lra \mathfrak B}**
which satisfies
~. Alternatively, the composition **Failed to parse (unknown function "\E"): {\displaystyle \E = \eta_s \circ P : \mathfrak A \lra \eta_s (B) \subset \mathfrak A}**
is a faithful conditional expectation.

### Groupoids and quantum compact groupoidsEdit

Let us consider to be a (topological) groupoid.

We denote by the space of smooth complex--valued functions with compact support on ~. In particular, for all , the function defined via convolution

is again an element of , where the convolution product defines the composition law on ~. We can turn into a *--algebra once we have defined the involution , and this is done by specifying ~.

#### Groupoid representationsEdit

We recall that following Landsman (1998) a *representation* of a groupoid **Failed to parse (unknown function "\grp"): {\displaystyle \grp}**
, consists of a family (or field) of Hilbert spaces
indexed by **Failed to parse (unknown function "\ob"): {\displaystyle X = \ob~ \grp}**
, along with a collection of maps **Failed to parse (unknown function "\grp"): {\displaystyle \{ U(\gamma)\}_{\gamma \in \grp}}**
, satisfying:

\item[1.]Failed to parse (unknown function "\lra"): {\displaystyle U(\gamma) : \mathcal H_{s(\gamma)} \lra \mathcal H_{r(\gamma)}}, is unitary. \item[2.] , wheneverFailed to parse (unknown function "\grp"): {\displaystyle (\gamma_1, \gamma_2) \in \grp^{(2)}}~ (the set of arrows). \item[3.] , for allFailed to parse (unknown function "\grp"): {\displaystyle \gamma \in \grp}~.

\subsubsection{Lie groupoids, their dual algebroids and representations on Hilbert space bundles}

Suppose now is a Lie groupoid. Then the isotropy group is a Lie group, and for a (left or right) Haar measure on , we can consider the Hilbert spaces as exemplifying the above sense of a representation. Putting aside some technical details which can be found in Connes (1994) and Landsman (2006), the overall idea is to define an operator of Hilbert spaces

Failed 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 \pi_x(f) : L^2(\mathsf{G_x},\mu_x) \lra L^2(\mathsf{G}_x, \mu_x)~, }

given by

for all , and
~. For each **Failed to parse (unknown function "\ob"): {\displaystyle x \in X =\ob ~\mathsf{G}}**
,
defines an involutive representation **Failed to parse (unknown function "\lra"): {\displaystyle \pi_x : C_c(\mathsf{G}) \lra \mathcal H_x<math>~. We can define a [[../NormInducedByInnerProduct/|norm]] on }**
C_c(\mathsf{G})</math> given by

whereby the completion of in this norm, defines
the reduced C*--algebra **Failed to parse (syntax error): {\displaystyle C^*_r(\mathsf{G )}**
of }. It is
perhaps the most commonly used C*--algebra for Lie groupoids
(groups) in noncommutative geometry.

#### Hilbert bimodules and tensor productsEdit

The next step requires a little familiarity with the theory of
Hilbert modules (see e.g. Lance, 1995). We define a left
--action and a right --action
on by and
~. For the sake of localization of the
intended Hilbert module, we implant a --valued inner product on given by P</math> is defined as a *completely positive map* .
Since is faithful, we fit a new norm on given by \mathfrak A</math> in this new
norm is denoted by leading then to a Hilbert
module over ~.

The tensor product \mathfrak B</math>,
which for , leads to *--homorphisms **Failed to parse (unknown function "\vp"): {\displaystyle \vp^{i} : \mathfrak A \lra \mathcal L_{\mathfrak B}(\mathfrak A^{-} \otimes \mathfrak A^{-})<math>~. Next is to define the (unital) C*--algebra }**
\mathfrak A
\otimes_{\mathfrak B} \mathfrak A</math> as the C*--algebra contained in
\vp^1(\mathfrak A)</math> and
**Failed to parse (unknown function "\vp"): {\displaystyle \vp^2(\mathfrak A)}**
~.

### Definition of compact quantum groupoids: axioms, coproducts, and bimodule antihomomorphismEdit

The last stage of the recipe for defining a
compact quantum groupoid entails considering a certain coproduct operation **Failed to parse (unknown function "\lra"): {\displaystyle \Delta : \mathfrak A \lra \mathfrak A \otimes_{\mathfrak B} \mathfrak A<math>, together with a coinverse }**
Q :
\mathfrak A \lra \mathfrak A</math> that it is both an algebra and
bimodule antihomomorphism. Finally, the following axiomatic
relationships are observed~:

Failed to parse (unknown function "\ID"): {\displaystyle (\ID \otimes_{\mathfrak B} \Delta) \circ \Delta &= (\Delta \otimes_{\mathfrak B} \ID) \circ \Delta \\ (\ID \otimes_{\mathfrak B} P) \circ \Delta &= P \\ \tau \circ (\Delta \otimes_{\mathfrak B} Q) \circ \Delta &= \Delta \circ Q }

where is a flip map : ~.

#### Locally compact quantum groupoids (LCQG)Edit

There is a natural extension of the above definition of quantum compact groupoids to *locally compact quantum groupoids* by taking to be a locally compact groupoid (instead of a compact groupoid), and then following the steps in the above construction with the topological groupoid being replaced by . Additional integrability and Haar measure system conditions need however be also satisfied as in the general case of locally compact groupoid *representations* .

## All SourcesEdit

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## ReferencesEdit

- ↑
E. M. Alfsen and F. W. Schultz:
*Geometry of State Spaces of Operator Algebras*, Birkh\"auser, Boston--Basel--Berlin (2003). - ↑
M. R. Buneci.:
*Groupoid Representations*, Ed. Mirton: Timishoara (2003). - ↑
J. M. G. Fell.: The Dual Spaces of C*--Algebras.,
*Transactions of the American Mathematical Society*,**94**: 365--403 (1960). - ↑
R. Gilmore:
*Lie Groups, Lie Algebras and Some of Their Applications.*, Dover Publs., Inc.: Mineola and New York, 2005. - ↑
P. Hahn: Haar measure for measure groupoids,
*Trans. Amer. Math. Soc*.**242**: 1--33(1978). - ↑
P. Hahn: The regular representations of measure groupoids.,
*Trans. Amer. Math. Soc*.**242**:34--72(1978).