nLab category algebra



Category theory




The space of functions 𝒞 1R\mathcal{C}_1 \to R on the space of morphisms 𝒞 1\mathcal{C}_1 of a small category 𝒞 \mathcal{C}_\bullet (with coefficients in some ring RR) naturally inherits a convolution algebra structure from the composition operation on morphisms. This is called the category convolution algebra or just category algebra for short.

Often this is considered specifically for groupoids and hence accordingly called groupoid convolution algebra or just groupoid algebra for short. (For one-object delooping groupoids of groups, groupoid algebras reduce to group algebras.) The inversion operation of the groupoid naturally makes its groupoid algebra into a star-algebra (this is generally the case for category algebras of dagger categories) and accordingly groupoid algebras play a role in C-star-algebra theory.

More generally, if the groupoid carries a line 2-bundle then (in its incarnation as a bundle gerbe-like transition bundle) the space of morphisms carries a line bundle (satisfying some compatibility conditions) and one can consider convolution algebras not just of functions, but of sections of this line bundle. The resulting algebra is called the twisted groupoid convolution algebra, twisted by the characteristic class of the line 2-bundle (TXLG).

For “bare” categories/groupoids (i.e.: internal to Set) these constructions are canonical. But under mild conditions or else when equipped some suitable extra structure, it generalizes to internal categories/internal groupoids in geometric contexts, notably in topology (topological groupoids), differential geometry (Lie groupoids), and algebraic geometry. In such geometric situations a groupoid convolution algebra equipped with its canonical coalgebra structure over the functions on its canonical atlas is also called a Hopf algebroid and may be used to characterize the geometric groupoid.

Therefore to some extent one may think of the relation between groupoids/categories and their groupoid/category algebras as an incarnation of the general duality between geometry and algebra. Since category/groupoid algebras are generically non-commutative, this relation identifies groupoids/categories as certain spaces in noncommutative geometry. From this point of view groupoid convolution algebras have been highlighted and developed notably in (Connes 94). Due to this relation the groupoid convolution product is also referred to as a star product and denoted “\star”. Groupoid C*-algebras form a rich sub-class of all C*-algebras, including crossed product C*-algebras, Cuntz algebras.

Groupoid convolution algebras may also be understood as generalizations of matrix algebras, to which they reduce for the case of the pair groupoid. In (Connes 94, 1.1) it was famously argued that when Werner Heisenberg (re-)discovered (infinite-dimensional) matrix algebras as algebras of observables in quantum mechanics, conceptually he rather considered groupoid convolution algebras. This perspective has since been fully developed: in (EH 06) strict deformation quantization is given fairly generally by twisted groupoid convolution algebras. See at geometric quantization of symplectic groupoids for more on this. For the discrete but higher geometry of infinity-Dijkgraaf-Witten theory quantization by higher groupoid convolution algebras is indicated in (FHLT 09).


For bare categories (discrete geometry)


Let 𝒞\mathcal{C} be a small category and let RR be a ring.

The category algebra or convolution algebra R[𝒞]R[\mathcal{C}] of 𝒞\mathcal{C} over RR is the RR-algebra

  • whose underlying RR-module is the free module R[𝒞 1]R[\mathcal{C}_1] over the set of morphisms of 𝒞\mathcal{C};

  • whose product operation is defined on basis-elements f,g𝒞 1R[𝒞]f,g \in \mathcal{C}_1 \hookrightarrow R[\mathcal{C}] to be their composition if they are composable and zero otherwise:

    fg:={gf ifcomposable 0 otherwise. f \cdot g := \left\lbrace \array{ g \circ f & if\;composable \\ 0 & otherwise } \right. \,.

We may identify elements in R[𝒞 1]R[\mathcal{C}_1] with functions 𝒞 1R\mathcal{C}_1 \to R with the property that they are non-vanishing only on finitely many elements. Under this identification for ϕ 1,ϕ 2\phi_1, \phi_2 two such functions, their product in R[𝒞]R[\mathcal{C}] is given by the formula

ϕ 1ϕ 2:f f 2f 1=fϕ 2(f 2)ϕ 1(f 1), \phi_1 \cdot \phi_2 \; \colon \; f \mapsto \sum_{f_2 \circ f_1 = f} \phi_2(f_2) \cdot \phi_1(f_1) \,,

where f,f 1,f 2𝒞 1f,f_1,f_2 \in \mathcal{C}_1. In particular if 𝒞\mathcal{C} is a groupoid so that every morphisms ff has an inverse f 1f^{-1} then this is equivalently

ϕ 1ϕ 2:f= g𝒞 1ϕ 2(fg 1)ϕ 1(g). \phi_1 \cdot \phi_2 \; \colon \; f \mapsto = \sum_{g \in \mathcal{C}_1} \phi_2(f \circ g^{-1}) \cdot \phi_1(g) \,.

This expresses convolution of functions.


If the small category 𝒞 \mathcal{C}_\bullet is a groupoid, hence equipped with an inversion map

inv:𝒞 1𝒞 1 inv \colon \mathcal{C}_1 \to \mathcal{C}_1

then pullback of functions along this map is an involution of the convolution algebra of 𝒞\mathcal{C} and hence makes it into a star-algebra.

More generally for 𝒞\mathcal{C} equipped with the structure of a dagger-category, pullback along the dagger-functor

:𝒞 1𝒞 1 \dagger \colon \mathcal{C}_1 \to \mathcal{C}_1

makes the convolution algebra a star-algebra.


If the category has finitely many objects and finitely many morphisms, then the category algebra is also a coalgebra via the unique comultiplication where every morphism in C 1R[C 1]C_1\subset R[C_1] is group like; the structure of the coalgebra and that of an algebra are compatible in the sense that they form a weak Hopf algebra.

For Lie groupoids

We discuss groupoid convolution C*-algebras for Lie groupoids/differentiable stacks (def. , prop. below).


If 𝒞\mathcal{C} is a groupoid with extra geometric structure, then there are natural variants of the above definition.

Notably if 𝒞\mathcal{C} is a Lie groupoid then there is a variant where the functions in remark are taken to be smooth functions and where the convolution sum is replaced by an integration. In order for this to make sense one needs to consider in fact functions with values in half-densities over the manifold 𝒞 1\mathcal{C}_1.

More generally, for a bundle gerbe over a Lie groupoid 𝒞\mathcal{C}, hence a multiplicative line bundle over 𝒞 1\mathcal{C}_1, one can consider a convolution product on sections of this line bundle tensored with half-densities.

See the References – For continuous/smooth geometry.


(Lie groupoid convolution algebra)

Let 𝒢 \mathcal{G}_\bullet be a Lie groupoid. Write C c (𝒢 1,Ω)C^\infty_c(\mathcal{G}_1, \sqrt{\Omega}) for the space of smooth half-densities in T ds=0Γ 1T dt=0Γ 1T_{d s = 0}\Gamma_1 \oplus T_{d t = 0}\Gamma_1 of compact support on its manifold 𝒢 1\mathcal{G}_1 of morphisms. Let the convolution product

:C c(𝒢 1,Ω)×C c(𝒢 1,Ω)C c(𝒢 1,Ω) \star \;\colon\; C_c(\mathcal{G}_1, \sqrt{\Omega}) \times C_c(\mathcal{G}_1, \sqrt{\Omega}) \to C_c(\mathcal{G}_1, \sqrt{\Omega})

be given on elements f,gC c(𝒢 1,Ω)f,g \in C_c(\mathcal{G}_1, \sqrt{\Omega}) over any element γ𝒢 1\gamma \in \mathcal{G}_1 by the integral

(fg):γ γ 2γ 1=γf(γ 1)f(γ 2). (f \star g) \colon \gamma \mapsto \int_{\gamma_2\circ \gamma_1 = \gamma} f(\gamma_1) \cdot f(\gamma_2) \,.

(Here we regard the integrand naturally as taking values in actual densities tensored with the pullback of Ω\sqrt{\Omega} along the composition map. This defines the integration of density-factor which then takes values in Ω\sqrt{\Omega}.)

The algebra (C c (𝒢 1,Ω),)(C_c^\infty(\mathcal{G}_1, \sqrt{\Omega}), \star) is the groupoid convolution algebra of smooth compactly supported functions. As in remark , this is naturally a star-algebra with involution inv *inv^\ast.

This construction originates around (Connes 82).


For 𝒢 \mathcal{G}_\bullet a Lie groupoid and for x𝒢 0x \in \mathcal{G}_0 any point in the manifold of objects there is an involutive representation π x\pi_x of the convolution algebra (C c (𝒢 1,Ω),,inv *)(C_c^\infty(\mathcal{G}_1, \sqrt{\Omega}), \star, inv^\ast) of def. on the canonical Hilbert space of half-densities L 2(s 1(x))L^2(s^{-1}(x)) of the source fiber of xx given on any ξL 2(s 1(x))\xi \in L^2(s^{-1}(x)) by

π x(f)ξ:γ γ 1t 1(γ)f(γ 1)ξ(γ 1 1γ). \pi_x(f) \xi \colon \gamma \mapsto \int_{\gamma_1 \in t^{-1}(\gamma)} f(\gamma_1) \xi(\gamma_1^{-1}\gamma) \,.

This defines a norm ||{\Vert \Vert} on the vector space C c (𝒢 1,Ω)C_c^\infty(\mathcal{G}_1, \Omega) given by the supremum of the norms in L 2(s 1(x))L^2(s^{-1}(x)) over all points xx:

fSup x𝒢 0π x(f). {\Vert f\Vert} \coloneqq Sup_{x \in \mathcal{G}_0} {\Vert \pi_x(f)\Vert} \,.

The Cauchy completion of the star algebra (C c (𝒢 1,Ω),,inv *)(C_c^\infty(\mathcal{G}_1, \sqrt{\Omega}), \star, inv^\ast) with respect to this norm is a C-star-algebra, the convolution C *C^\ast-algebra of the Lie groupoid 𝒢 \mathcal{G}_\bullet.

This is recalled at (Connes 94, prop. 3 on p. 106).


With suitable definitions, this construction constitutes something at least close to a 2-functor from differentiable stacks to C-star-algebras and Hilbert bimodules between them:

In (Mrčun 99) the convolution algebra construction for étale Lie groupoids is extended to groupoid bibundles and shown to produce a functor to C-star-algebras with (isomorphism classes of) bimodules between them. In (Kališnik-Mrčun 07) it is shown that if one remembers the additional Hopf algebroid structure on the convolution C *C^\ast-alegras (the algebraic analog of remebering the atlas of the differentiable stack of a Lie groupoid) then this construction becomes a full subcategory inclusion of étale Lie groupoids into their convolution Hopf algebroids.

In (Muhly-Renault-Williams 87, Landsman 00) the generalization of the construction of a C *C^\ast-bimodule from a groupoid bibundle to general Lie groupoids is discussed (not necessarily étale), but only equivalence-bibundles are considered and are shown to yield Morita equivalence bimodules (no discussion of composition and functoriality here).

A precise form of this statement is the following Nuiten 13, theorem 3.3.1


(groupoid convolution algebra is (2,1)-functor from differentiable stacks to C*-algebras)

The above construction of groupoid convolution algebras (def. ) extends to a (2,1)-functor

C *():DiffStack propAAAC *Alg bim op C^\ast(-) \;\colon\; DiffStack^{prop} \overset{\phantom{AAA}}{\longrightarrow} C^\ast Alg^{op}_{bim}


  1. the (2,1)-category DiffStack propDiffStack_{prop} of differentiable stacks (i.e. Lie groupoids regarded as smooth stacks) with proper morphisms between them (Nuiten 13, def. 2.2.35)

  2. the opposite (2,1)-category of C*-algebras with Hilbert bimodules between them and intertwiners between those.

(Nuiten 13, theorem 3.3.1)

This way, much of noncommutative geometry is exhibited as actually being the higher geometry of differentiable stacks inside all smooth stacks. In particular, groupoid K-theory is realized as the KK-theory of convolution C-algebras.

Equivalent characterizations

We discuss equivalent characterizations of category algebras/groupoid algebras that are useful in certain context

As a weak colimit over a constant 2Vect2Vect-valued functor

Apparently for 𝒞\mathcal{C} a groupoid the category algebra of CC is the weak colimit over 𝒞\mathcal{C} of the functor 𝒞Vect-Mod\mathcal{C} \to Vect\text{-}Mod constant on the ground field algebra.

This statement is for instance in (FHLT, section 8.4).

The 2-cell in the universal co-cone corresponding to the morphism fCf \in C is the k-k[C]k\text{-}k[C]-bimodule homomorphism f():AAf \cdot (-) : A \to A that multiplies by fk[C]f \in k[C] from the left.

x f y k k k k[C] f() k[C] k[C] \array{ x &&\stackrel{f}{\to}&& y \\ k &&\stackrel{k}{\to}&& k \\ & {k[C]}_{\mathllap{}}\searrow &\swArrow_{f \cdot (-)}& \swarrow_{\mathrlap{k[C]}} \\ && k[C] }

This description should be compared with the analogous description of the action groupoid by a weak colimit. One sees that the groupoid algebra is a linear incarnation of the action groupoid in some sense.

In terms of composition of spans

The category algebra of a category CC is a special case of a general construction of spans (see also at bi-brane).

In order not to get distracted by inessential technicalities, consider the case of a finite category CC, i.e. an internal category in FinSet. This is a span

C 1 s t C 0 C 0 \array{ && C_1 \\ & {}^{s}\swarrow && \searrow^{t} \\ C_0 &&&& C_0 }

equipped with a composition operation: a morphism of spans from the composite span

C 1× t,sC 1 C 1 C 1 s t s t C 0 C 0 C 0 \array{ &&&& C_1 \times_{t,s} C_1 \\ &&& \swarrow && \searrow \\ && C_1 &&&& C_1 \\ & {}^{s}\swarrow && \searrow^{t} && {}^{s}\swarrow && \searrow^{t} \\ C_0 &&&& C_0 &&&& C_0 }

to the original one, i.e. a morphism

comp:C 1× t,sC 1C 1 comp : C_1 \times_{t,s} C_1 \to C_1

which respects source and target morphisms.

Given this, consider the trivial vector bundle on the set of objects C 0C_0. This is nothing but an assignment

I:C 0Vect k I : C_0 \to Vect_k

of the ground field kk to each element of C 0C_0. There are two different ways to pull this vector bundle on objects back to a vector bundle on morphisms, once along the source, once along the target map.

Then notice that the set of natural transformations between these two vector bundles

Hom [Sets,Vect k](s *I,t *I) Hom_{[Sets,Vect_k]}(s^* I , t^* I)

whose elements are 2-arrows of the form

C 1 s t C 0 f C 0 I I Vect k \array{ && C_1 \\ & {}^{s}\swarrow && \searrow^{t} \\ C_0 &&\stackrel{f}{\Rightarrow}&& C_0 \\ & {}_I \searrow && \swarrow_I \\ && Vect_k }

are canonically in bijection with kk-calued functions on C 1C_1, hence with the vector space spanned by C 1C_1, hence with the vector space underlying the category algebra

Hom [Sets,Vect k](s *I,t *I)k[C]. Hom_{[Sets,Vect_k]}(s^* I , t^* I) \simeq k[C] \,.

The algebra structure on k[C]k[C] is similarly encoded in the diagrammatics: given two elements

C 1 s t C 0 f C 0 I I Vect kand C 1 s t C 0 g C 0 I I Vect k \array{ && C_1 \\ & {}^{s}\swarrow && \searrow^{t} \\ C_0 &&\stackrel{f}{\Rightarrow}&& C_0 \\ & {}_I \searrow && \swarrow_I \\ && Vect_k } \;\;\;\; and \;\;\;\; \array{ && C_1 \\ & {}^{s}\swarrow && \searrow^{t} \\ C_0 &&\stackrel{g}{\Rightarrow}&& C_0 \\ & {}_I \searrow && \swarrow_I \\ && Vect_k }

their pre-composite is the diagram

C 1× t,sC 1 C 1 C 1 s t s t C 0 f C 0 g C 0 Vect k . \array{ &&&& C_1 \times_{t,s} C_1 \\ &&& \swarrow && \searrow \\ && C_1 &&&& C_1 \\ & {}^{s}\swarrow && \searrow^{t} && {}^{s}\swarrow && \searrow^{t} \\ C_0 &&\stackrel{f}{\Rightarrow}&& C_0 &&\stackrel{g}{\Rightarrow}&& C_0 \\ & \searrow &&& \downarrow &&& \swarrow \\ && \to &&Vect_k&& \leftarrow } \,.

This is a composite transformation between three trivial vector bundles on the set C 1× t,sC 1C_1 \times_{t,s} C_1 of composable morphisms in CC. As such, it is a function, which on the element consisting of the composable pair rs\stackrel{r}{\to}\stackrel{s}{\to} takes the value f(r)g(s)f(r)\cdot g(s).

In order to get back a transformation between vector bundles on C 1C_1, hence a transformation between vector bundles on C 1C_1, we push forward along the composition map comp:C 1× t,sC 1C 1comp: C_1 \times_{t,s} C_1 \to C_1. This just means that we add up the values on the fibers of this map.

The result is the convolution product

(fg):t sr=tf(r)g(s). (f\star g) : t \mapsto \sum_{s\circ r = t} f(r)\cdot g(s) \,.

This is indeed the product in the category algebra.

Looking at category algebras realizes them as a puny special case of a bigger story which involves bi-branes as morphisms between nn-bundles/(n1)(n-1)-gerbes which live on spaces connected by correspondence spaces. This is related to a bunch of things, such as T-duality, Fourier-Mukai transformations and other issues of quantization. A description of this perspective is in

This is related to observations such as described here:


Basic examples


The convolution algebra of a set/manifold XX regarded as a discrete groupoid/Lie groupoid with only identity morphisms is the ordinary function algebra of XX.


For XX a set the convolution algebra of the pair groupoid Pair(X) Pair(X)_\bullet is the matrix algebra of |X|×|X|{\vert X\vert} \times {\vert X\vert} matrices.


For XX a smooth manifold and Pair(X) Pair(X)_\bullet its pair groupoid regarded as a Lie groupoid its smooth convolution algebra is the algebra of smoothing kernels on XX.


𝒞=BG\mathcal{C} = \mathbf{B}G is the delooping groupoid of a discrete group GG (the groupoid with a single object and GG as its set of morphisms), then def. reduces to that of the group algebra of GG:

R[BG]R[G]. R[\mathbf{B}G] \simeq R[G] \,.

Incidence algebras (poset convolution algebras)

See at Möbius inversion#IncidenceAlgebrasZetaFn.

Higher groupoid convolution algebras and n-vector spaces/n-modules

under construction

We discuss here a natural generalization of the notion of groupoid convolution algebras to higher algebras for higher groupoids.

There may be several sensible such generalizations. The one discussed now follows the principle of iterated internalization and naturally matches to the concept of n-modules (n-vector spaces) as they appear in extended prequantum field theory.

In order to disentangle conceptual from technical aspects, we first discuss geometrically discrete higher groupoids. The results of this discussion then in particular help to suggest what the right definition of “higher Lie groupoid” in the context of higher convolution algebras should be in the first place.

The considerations are based on the following


By the discussion at 2-module we may think of the 2-category kAlg bk Alg_b of kk-associative algebras and bimodules between them as a model for the 2-category 2Mod of kk-2-modules that admit a 2-basis (2-vector spaces). Hence the groupoid convolution algebra constructiuon is a 2-functor

C:Grpd2Mod. C \;\colon\; Grpd \to 2 Mod \,.

There is then the following systematic refinement of this to higher groupoids and higher algebra: by the discussion at n-module, 3-modules are algebra objects in 2Mod and maps between them are bimodule objects in there. An algebra object in kAlg bk Alg_b is equivalently a sesquialgebra, an algebra equipped with a second algebra structure up to coherent homotopy, that is exhibited by structure bimodules.

Special cases of this are bialgebras, for which these structure bimodules come from actual algebra homomorphisms. Examples of these in turn are Hopf algebras. These we naturally re-discover as special higher groupoid convolution higher algebras in example below.

This iterated internalization on the codomain of the groupoid convolution algebra functor has a natural analog on its domain: a 2-groupoid we may present by a double groupoid, namely a groupoid object in an (∞,1)-category in Grpd which is 3-coskeletal as a simplicial object in Grpd.


Given a groupoid object 𝒢 \mathcal{G}_\bullet in the (2,1)-topos Grpd hence a double groupoid, applying the groupoid convolution algebra (2,1)(2,1)-functor CC to the corresponding simplicial object 𝒢 Grpd Δ op\mathcal{G}_\bullet \in Grpd^{\Delta^{op}} yields:

  • groupoid convolution algebras C(𝒢 0)C(\mathcal{G}_0) and C(𝒢 1)C(\mathcal{G}_1),

  • a C(𝒢 1) C(𝒢 0,1)C(𝒢 1)C(𝒢 0)C(\mathcal{G}_1) \otimes_{C(\mathcal{G}_{0,1})} C(\mathcal{G}_1)-C(\mathcal{G}_{0})-bimodule, assigned to the composition functor 1:𝒢 1×𝒢 0𝒢 1𝒢 1\partial_1 \colon \mathcal{G}_1 \underset{\mathcal{G}_0}{\times} \mathcal{G}_1 \to \mathcal{G}_1.

Under the 2-functoriality of CC, the Segal conditions satisfied by 𝒢 \mathcal{G}_\bullet make this bimodule exhibit a sesquialgebra structure over C(𝒢 0,1)C(\mathcal{G}_{0,1}).

This sesquialgebra we call the the double groupoid convolution 2-algebra of 𝒢 \mathcal{G}_\bullet.

(Here we make invariant sense of the tensor product by evaluating on a Reedy fibrant representative.)


Let GG be a finite group. Write BG\mathbf{B}G for its delooping groupoid (the connected groupoid with π 1=G\pi_1 = G). Since this is just a 1-groupoid, there are two natural ways to present BG\mathbf{B}G as a double groupoid:

  1. lim(BGBGididBG)BG\underset{\longrightarrow}{\lim}(\cdots \mathbf{B}G\stackrel{\to}{\stackrel{\to}{\to}} \mathbf{B}G \stackrel{\overset{id}{\to}}{\underset{id}{\to}} \mathbf{B}G) \simeq \mathbf{B}G;

  2. lim(G×GG*)BG\underset{\longrightarrow}{\lim}(\cdots G \times G \stackrel{\to}{\stackrel{\to}{\to}} G \stackrel{\to}{\to} *) \simeq \mathbf{B}G.

(The first is “vertically constant”, the second is “horizontally constant”).

Applying the groupoid convolution algebra functor to the first presentation yields the groupoid convolution algebra C(BG)C(\mathbf{B}G) equipped with a trivial multiplication bimodule, hence just the group convolution algebra C(BG)C conv(G)C(\mathbf{B}G) \simeq C_{conv}(G).

Applying however the groupoid convolution algebra functor to the second presentation yields the commutative algebra of functions C(G)C(G) equipped with the multiplication bimodule which is C(G×G)C(G \times G) regarded as a (C(G×G),C(G))(C(G\times G), C(G))-bimdodule, where the right action is induced by pullback along the group product map G×GGG \times G \to G.

This bimodule is in the image of the functor AlgAlg bAlg \to Alg_b that sends algebra homomorphisms to their induced bimodules, by sending f:ABf \colon A \to B to AA regarded as an (A,B)(A,B)-bimodule with the canonical left action on itself and the right action induced by ff. Namely this bimdoule correspondonds to the map

Δ:C(G)C(G×G)C(G)C(G) \Delta \colon C(G) \to C(G \times G) \simeq C(G) \otimes C(G)

given on ϕC(G)\phi \in C(G) and g 1,g 2Gg_1, g_2 \in G by

Δϕ:(g 1,g 2)ϕ(g 1g 2). \Delta \phi \colon (g_1, g_2) \mapsto \phi(g_1 \cdot g_2) \,.

This is that standard coproduct on the standard dual Hopf algebra associated with GG.

In summary this means that (for GG a finite group):

  1. If we regard BG\mathbf{B}G as presented as a double groupoid constant on BG\mathbf{B}G, then the corresponding groupoid convolution sesquialgebra (basis for a 3-module) is the convolution algebra of GG;

  2. If instead we regard BG\mathbf{B}G as presented as the double groupoid which is degreewise constant as a groupoid, then the corresponding groupoid convolution sesquialgebra is the standard (“dual”) Hopf algebra structure on the commutative pointwise product algebra of functions on GG.


Relation to (twisted) K-theory

The operator K-theory of the convolution C *C^\ast-algebra of a topological groupoid 𝒳 \mathcal{X}_\bullet may be thought of as the topological K-theory of the corresponding topological stack. More generally, for 𝒳B 2U(1)\mathcal{X} \to \mathbf{B}^2 U(1) a principal 2-bundle (bundle gerbe) on the groupoid/stack, the operator K-theory of the corresponding twisted convolution algebra is the twisted K-theory of the stack.

(Tu, Xu, Laurent-Gengoux 04)


For partial orders

  • Gian-Carlo Rota, On the foundations of combinatorial theory I: theory of Möbius functions , Z. Wahrscheinlichkeitstheorie und Verw. Gebiete 2 (1964), 340–368.

For discrete geometry

The homotopy colimit-interpretation of category algebras over discrete categories is discussed in

Groupoid algebras of geometrically discrete groupoids twisted by principal 2-bundles/bundle gerbes/groupoid central extension is reviewed in

  • Eitan Angel, A Geometric Construction of Cyclic Cocycles on Twisted Convolution Algebras, PhD thesis (2010)

    Cyclic cocycles on twisted convolution algebras, (arXiv.1103.0578)

For continuous/smooth geometry

Convolution C *C^\ast-algebras

The study of convolution C-star algebras of Lie groupoids goes back to

  • Jean Renault, A groupoid approach to C *C^\ast algebras, Springer Lecture Notes in Mathematics 793, Springer (1980)

Where the integration is performed against a fixed Haar measure.


The construction via sections of bundles of half-densities (avoiding a choice of Haar measure) is due to

  • Alain Connes, A survey of foliations and operator algebras, Proc. Sympos. Pure Math., AMS Providence, 32 (1982), 521–628


Groupoid algebras as strict deformation quantization of Lie-Poisson structures given by Lie algebroids:

Specifically of symplectic groupoids in the context of geometric quantization of symplectic groupoids

See also:

More along these lines is in

  • Paul Muhly, Dana P. Williams, Continuous trace groupoid C *C^\ast-algebras II Math. Scand. 70 (1992), no. 1, 127–145; MR 93i:46117). (pdf)

  • Paul Muhly, Jean Renault, Dana P. Williams, Continuous trace groupoid C *C^\ast-algebras III , Transactions of the AMS, vol 348, Number 9 (1996) (jstor)

  • Mădălina Roxana Buneci, Groupoid Representations, Ed. Mirton: Timishoara (2003).

  • Mădălina Roxana Buneci, Groupoid C *C^\ast-Algebras, Surveys in Mathematics and its Applications, Volume 1: 71–98. (pdf)

  • Mădălina Roxana Buneci, Convolution algebras for topological groupoids with locally compact fibers (2011) (pdf)

A review in the context of geometric quantization is in section 4.3 of

  • Rogier Bos, Groupoids in geometric quantization PhD Thesis (2007) pdf

Specifically the convolution C *C^\ast-algebras of bundle gerbes regarded as centrally extended groupoids (algebras whose modules (see below) are gerbe modules/twisted bundle) is discussed in section 5 of

Functoriality of the construction of C *C^\ast-convolution algebras (its extension to groupoid-bibundles) is discussed in

  • Paul Muhly, Jean Renault, D. Williams, Equivalence and isomorphism for groupoid C *C^\ast-algebras, J. Operator Theory 17 (1987), no. 1, 3–22.
  • Janez Mrčun, Functoriality of the bimodule associated to a Hilsum-Skandalis map. K-Theory 18 (1999) 235–253.

  • Klaas Landsman, The Muhly-Renault-Williams theorem for Lie groupoids and its classical counterpart, Lett. Math. Phys. 54 (2000), no. 1, 43–59. (arXiv:math-ph/0008005)

  • Klaas Landsman, Operator Algebras and Poisson Manifolds Associated to Groupoids, Commun. Math. Phys. 222, 97 – 116 (2001) (web)

Convolution Hopf algebroids

A characterization of the convolution algebras of étale groupoids with their Hopf algebroid structure is in

Modules over Lie groupoid convolution algebras and K-theory

Discussion of modules over Lie groupoid convolution algebras is in the following articles.

In (Renault80) measurable representations of topological groupoids are related to modules over their L 1L^1 convolution star algebra Banach algebras hence over their envoloping C *C^\ast-algebras.

In (Bos, chapter 7) is discussion refining this to continuous representations and representation of a convolution C *C^\ast-algebra, also in section 4 of:

Representation of convolution algebras of étale groupoids is in

The operator K-theory of groupoid convolution algebras (the topological K-theory of the corresponding differentiable stacks) is discussed in

Construction of cocycles in KK-theory and spectral triples from groupoid convolution is in

Last revised on December 4, 2023 at 16:52:11. See the history of this page for a list of all contributions to it.