# Non-commutative duality I: Which C*-algebras are (isomorphic to a) convolution algebra?

Many interesting C*-algebras can be realized as convolution algebras over a groupoid, an idea introduced in 1980 by Jean Renault (this entry in nLab provides plenty of context to the general approach of attaching an algebra to a groupoid).

Perhaps due to my incompetence in this formidable field, I was not able to identify some results in the way of characterizing which C*-algebras are in fact convolution algebras.

So, here is my question:

Is there a duality theorem stating that the sub-category of C*-algebras satisfying (add here some properties) is dual to the category of locally compact groupoids ?

ADDENDUM: I have changed the title to make clear what I said in my comments below: though this is a very specific question (and by no means trivial, see the excellent responses below), it is just a tassel toward a broad goal, namely get some kind of duality theory for non commutative spaces. I was (happily) surprised not only with the answers I have received, but also by the views and the likes this question has gotten. That means that numerous folks share this interest. There will be further questions under the same rubric, and I hope they will attract other members of MO from a broad variety of backgrounds (operator theory, higher category theory, mathematical physicists, and generally researchers with a passion for duality theory)

• I'm pretty sure it is known that there are $C^*$-algebras not arising from a groupoid, even up to Morita equivalence, but I can't quite remember who that paper was written by. In the paper it remarks on a property that groupoid $C^*$-algebras have (to do with something induced by the inverse map) that the example constructed didn't have this property. If I find the paper, I'll drop a reference. Oct 3, 2020 at 10:27
• Aha, here it is arxiv.org/abs/1708.04105v1: "We show in this paper that not every C∗-algebra has a groupoid model. We achieve this by showing that all groupoid C∗-algebras are self-opposite in the sense that they are isomorphic to their opposite C∗-algebras." So, a necessary condition, though not (yet) sufficient. But also note: "Although our result implies the existence of C∗-algebras with no groupoid model, it is still possible that such C∗-algebras can be realised as twisted groupoid C∗-algebras." Oct 3, 2020 at 10:30
• Thanks David, I appreciate ! That goes a long way toward what I need.... Oct 3, 2020 at 10:32
• The name of @DavidRoberts's reference: Buss and Sims - Opposite algebras of groupoid C*-algebras. Oct 3, 2020 at 14:23

The main obstruction to this kind of duality is not so much that not every $$C^*$$-algebra is a convolution algebra (though, at least if we don't use twisted convolution algebra, there are known obstruction as mentioned in the comment), but rather that the construction that attach a convolution $$C^*$$-algebra to a groupoid is not 'injective' at all.

This is mostly due to what I want to call "Fourier isomorphisms" between $$C^*$$-algebras, that exists at the analytic level without having a clear geometric origin (well, I'm sure one can find a geometric explanation for Fourier duality, but what I mean is that it cannot be interpreted as a morphisms or bibundle between the groupoids).

A typical example: $$C^*(B \mathbb{Z}) \simeq C(\mathbb{U})$$

Take the groupoid $$B\mathbb{Z}$$ with a single object $$*$$ and the additive groupe $$\mathbb{Z}$$ as its automorphism group. We see it as a topological groupoid for the discrete topology.

The associated $$C^*$$-algebra (both maximal and reduced) $$C^*(B\mathbb{Z})$$ is simply the group $$C^*$$-algebra of $$\mathbb{Z}$$. Its a commutative $$C^*$$-algebra, by Gelfand duality it is isomorphic to continuous fonction on its spectrum. Here it is the algebra of continuous function on $$\mathbb{U}$$ the unit circle (the elements $$n \in \mathbb{Z}$$ corresponds to the function $$z \mapsto z^n$$).

But I can also considered the groupoid with $$\mathbb{U}$$ as set of objects and no non-identity morphisms, that I topologize with the topology of $$\mathbb{U}$$. The $$C^*$$-algebra attached to this groupoid are simply continuous function on $$\mathbb{U}$$, hence the same $$C^*$$-algebra as before.

So, if you want to recover the $$C^*$$-algebra you need some additional structure on it, not just a property. For example the notion of "Cartan subalgebra", which represent the subalgebra of the convolution algebra of continuous function on $$G_0$$ does the trick in some cases. Have a look at Cartan Subalgebras in $$C^*$$-Algebras by Jean Renault for example, the paper also cite other similar result in different context.

• This is a great answer. Oct 3, 2020 at 14:08
• @MircoA.Mannucci : The reason I talked about fourier transform is because the example generalizes to any locally compact topological group $G$: $C^*(BG) \simeq C(G^*)$ where $G^*$ is the pontryagin dual, and this isomorphism is the Fourier duality for the group $G$. Even for more general groupoids, these kind of isomorphisms often feels like some kind of genealized Fourier transform. I don't think you can define a Fourier dual of a $C^*$-algebra in general, but there is definitely a notion of Fourier dual for $C^*$-algebra with additional structure, see for e.g. the theory of Quantum groups Oct 3, 2020 at 16:19
• (...one kind of Quantum groups are some sort of $C^*$-Hopf algebra, and there is a Fourier duality theory for these, there are also purely algebraic notion of Quantum group, but that's not what I was refering too) Oct 3, 2020 at 16:20
• Absolutely. Convolution algebra have a lot of additional structure. The most documented example are these cartan subalgebra I mentioned in my original answer (for which there are partial result in the spirit of a n.c. duality) . But there are many other kind of structure. For example, the category of representation of a convolution algebra can be described in terms of representation of the groupoids and inherits a monoidal structure because of this. That does not quite corresponds to a bialgebra structure on the $C^*$-algebra, but that's something in this direction. Oct 3, 2020 at 16:51
• There is a sort of duality between etale groupoids with totally disconnected unit space and boolean inverse semigroups. The Exel C*-algebra of the boolean inverse semigroup is the groupoid C*-algebra. I'm not sure how much further one can take this among groupoids even in the etale case Oct 3, 2020 at 17:48

As already pointed out, Buss and Sims have found an example of a $$C^*$$-algebra which is not isomorphic to its opposite, and hence it is not a groupoid $$C^*$$-algebra. However twisted groupoid $$C^*$$-algebras are not necessarily self-opposite so, as the authors point out, nothing so far prevents their example from being realized as a twisted convolution algebra. In fact, it appears that nobody knows an example of a $$C^*$$-algebra which is not a twisted convolution algebra!!

Among the algebras known to be (twisted or untwisted) convolution algebras one finds all Kirchberg algebras satisfying the UCT as well as all algebras in the Elliott classification program, including the somewhat elusive Jiang-Su algebra $$\mathscr Z$$.

On the other hand, the vast majority of useful tools available to the study of (untwisted) groupoid $$C^*$$-algebras apply just as well for twisted ones, so perhaps the more relevant question is which $$C^*$$-algebras are isomorphic to a twisted convolution algebra.

As mentioned by @Simon, the best results so far leading to a "Fourier-like" decomposition of a $$C^*$$-algebra as a convolution algebra (of an etale groupoid) are based on the assumption that we already know the abelian subalgebra of functions supported on the object space. This abelian subalgebra is sometimes called a Cartan algebra, given the similarities with the homonymous concept from the theory of Lie groups.

One of the stickiest points in this subject is whether or not there exists a conditional expectation onto the given Cartan subalgebra (one of the main assumptions in Renault's result). The reason one might prefer not to require this condition is that there are many groupoids whose convolution algebra does not admit such a conditional expectation due to the fact that the underlying groupoid is not Hausdorff. Examples of this situation are very common such as groupoids arising from foliations and certain dynamical systems.

Another relevant question is whether or not the Cartan subalgebra is maximal abelian, a crucial feature in the Lie algebra version of this concept, as well as an important assumption in Renault's theory. Maximal commutativity is closely related to topological freenes of the associated groupoid (a concept borrowed from those dynamical systems in which most points have trivial isotropy group). In particular, in the example given by @Simon of $$C^*(B\mathbb Z)$$, the natural Cartan subalgebra is $$\mathbb C$$, which is clearly not maximal commutative at the same time that the action of the group $$\mathbb Z$$ on a point has too much isotropy!

Working with David Pitts (Characterizing groupoid C*-algebras of non-Hausdorff étale groupoids, arXiv:1901.09683) we have found a characterization of twisted groupoid $$C^*$$-algebras which neither assumes conditional expectations nor maximal commutativity. Perhaps one of the simplest examples where our result applies is $$C(S^1)\subseteq C([0,1])$$ where we identify $$C(S^1)$$ as the subalgebra of periodic functions. Surprisingly, there is a twisted groupoid description of this "Cartan pair".

Significantly, the lack of a conditional expectation from $$C([0,1])$$ to $$C(S^1)$$ prevents the groupoid from being Hausdorff. In fact, the topological space underlying this groupoid is essentially the best known example of a non-Hausdorff topological space in which one takes the unit interval $$[0,1]$$ with a duplicate copy of the point 1, except that we instead duplicate a chosen point of $$S^1$$. The groupoid structure is such that the two duplicate points form a copy of the 2-element group, while all other points are considered to be objects. The twist is non trivial but it turns out to be the most natural nontrivial twist one can think of.

• When I was a young grad student I was (like everyone else) trying to show that I understood everything. Now, Praise the Lord, I am happy to throw a pebble in the water and hear such talented people 's voices, even though they make me feel like an idiot. It will take me a week, perhaps more, to digest your answer. Meanwhile, please come back! PS looks like everything is converging somewhere... Oct 3, 2020 at 19:55
• @Mirco, I am sure if you came across a question sitting squarely in your area of expertise, you would do even better!
– Ruy
Oct 3, 2020 at 20:00
• Regarding your last comment there is a fundamental difficulty in making such a duality theory work because groupoids generalize both topological spaces and groups, while the natural dualities between each of these categories and C*-algebras are contra-variant for spaces and covariant for groups. On the other hand I think Xin Li has done some advances on figuring this issue out.
– Ruy
Oct 3, 2020 at 20:13
• Regarding twists, I am not sure I understood your question, but a twisted groupoid is a pair $(\mathscr G,\Gamma)$, where $\mathscr G$ is a groupoid, and $\Gamma$ is a twist. In other words, the groupoid exists irrespectively of the twist. I'll try to get a few references in case you want to read about it.
– Ruy
Oct 3, 2020 at 20:18
• @Mirco, of course the main reference for groupoids is Renault's thesis (LNM 793), where he defines twists as the natural generalization of 2-cocycles for groups. In 1986 Kumjian (Can. J. Math.) came up with a more general notion based on groupoid extensions, which is strictly more general then Renault's and the appropriate one for the discussion we are having here. Kumjian later reformulated his point of view in terms of line bundles, the best point of view in my opinion and this can be found in his paper "Fell bundles over groupoids”, Proc. Amer. Math. Soc., 126 (1998).
– Ruy
Oct 3, 2020 at 22:19

There are already excellent answer so I hope my small remarks here will not seem to be too trivial. They concern the geometric origin, in a way, of the Fourier morphism mentioned by Simon Henry in a very specific contect.

When quantizing Poisson manifolds into NC $$C^*$$-algebras on possible way is through what is called groupoid quantization. The procedure is, here, to build the symplectic groupoid integrating the original Poisson manifold (thus a very specific kind of Lie groupoid) and then performing geometric quantization on this symplectic manifold keeping in mind, in a way, its groupoid structure.

The relevant part is the choice of a polarization compatible with the groupoid structure: what is called a multiplicative polarization. Such polarizations induce a groupoid fibration from the original symplectic groupoid to a quotient groupoid (which is not Lie but only topological) and a cocycle on this quotient groupoid coming from the symplectic structure. The (twisted by the cocycle) groupoid $$C^*$$ algebra thus resulting is the outcome of the groupoid quantization procedure.

Different choice of polarizations may result in quotient groupoids much differing one from the other and in the quotient cocylce being trivial or not. The typical example is an invariant symplectic structure $$\omega_\theta$$ ($$\theta\not\in \mathbb Q$$) on the torus which, depending on the choice of polarization, may be either quantized by the irrational rotation algebra (groupoid $$\mathbb Z\ltimes_\theta \mathbb S^1$$ with trivial cocycle) or by a trivial groupoid based on $$\mathbb R^2$$ (0-isotropy) but with non trivial cocycle depending on $$\theta$$. That the two are isomorphic may be considered a quantization does not depend on polarization type of result. The relation between the two is a partial Fourier transform applied to one of the two variables, in a way, and in general whenever the symplectic groupoid can be identified with a cotangent bundle as a manifold (which happens for a wide class of Poisson manifolds) different choices of polarization defines some Fourier-type relation between different (twisted) groupoid $$C^*$$-algebras.

One of the subtle points here is that in some sense groupoid $$C^*$$-algebras behave in a contravariant functorial way with respect to the base and in a covariant functorial way with respect to the isotropy so that all relations (like this choice of polarization) that result in some interchange of variables between base and isotropy have a quite complicated functorial description.

• Eli Hawkins, A groupoid approach to quantization, J. Symplectic Geom. 6 Number 1 (2008) 61–125. Project Euclid, arXiv:math/0612363.
• first of all, thanks for your contribution! here the horizons expand, and that is a good thing. But I want to understand, and, to this effect, I would ask you to schematize what you have said. I do not mean to add technicalities, proofs, etc. Only the key passages. Let me start and then you correct me, and/or fill the details. Oct 4, 2020 at 18:10
• so, 1) one starts from a (classical ) Poisson Manifold and its simplectic groupoid so far, so good. Now I am already lost: 2) you build a quotient groupoid (of the first one) which is only topological . I do not understand how that is done, and need clarification. I assume that , assuming the second groupoid is done, 3 ) one looks at TWO C* algebra, one from groupoid 1 and the other from its quotient. If I follow you, there is a (knd of) Fourier transform between these two. Oct 4, 2020 at 18:16
• PS I looked into your profile and I found a question of yours which is EXTREMELY relevant to me, namely the functoriality of the C*algebra for the cat of graphs.... Oct 4, 2020 at 18:28
• 2. You choose polarization, i.e.a Lagrangian distribution with the additional property that the space of leaves inherits a groupoid structure. The quotient map is now only topological. Oct 4, 2020 at 21:18
• 3. different choices of polarization results in different quotient groupoids. In some cases you can understand the relation between such groupoids as a Fourier transform. Oct 4, 2020 at 21:19