
The radial MASA in free orthogonal quantum groups (with R. Vergnioux), J. Funct. Anal. 271 (2016), n^{o} 10, 27762807: link and pdf (old version).
Abstract
We prove that the radial subalgebra in free orthogonal quantum group factors is maximal abelian and mixing,
and we compute the associated bimodule.
The proof relies on new properties of the JonesWenzl projections and on an estimate of certain scalar products of coefficients of irreducible representations.

Wreath products of finite groups by quantum groups (with A. Skalski), to appear in J. Noncommut. Geom. : link and pdf (old version).
Abstract
We introduce a notion of partition wreath product of a finite group by a partition quantum group,
a construction motivated on the one hand by classical wreath products and on the other hand by the free wreath product of J. Bichon.
We identify the resulting quantum group in several cases,
establish some of its properties and show that when the finite group in question is abelian, the partition wreath product is itself a partition quantum group.
This allows us to compute its representation theory, using earlier results of the first named author.

On the partition approach to SchurWeyl duality and free quantum groups (with an appendix by A. Chirvasitu), Transform. Groups 22 (2017), n^{o} 3, 707751 : link and pdf (old version).
Abstract
We give a general definition of classical and quantum groups whose representation theory is "determined by partitions" and study their structure.
This encompasses many examples of classical groups for which SchurWeyl duality is described with diagram algebras
as well as generalizations of P. Deligne's interpolated categories of representations.
Our setting is inspired by many previous works on easy quantum groups and appears to be wellsuited to the study of free fusion semirings.
We classify free fusion semirings and prove that they can always be realized through our construction,
thus solving several open questions.
This suggests a general decomposition result for free quantum groups which in turn gives information on the compact groups whose
SchurWeyl duality is implemented by partitions.
The paper also contains an appendix by A. Chirvasitu proving simplicity results for the reduced C*algebras of some free quantum groups.

On bifree De Finetti theorems (with M. Weber), Ann. Math. Blaise Pascal 23 (2016), n^{o} 1, 2151 : link and pdf (old version).
Abstract
We investigate possible generalizations of the de Finetti theorem to bifree probability.
We first introduce a twisted action of the quantum permutation groups corresponding to the combinatorics of bifreeness.
We then study properties of families of pairs of variables which are invariant under this action,
both in the binoncommutative setting and in the usual noncommutative setting.
We do not have a completely satisfying analogue of the de Finetti theorem,
but we have partial results leading the way. We end with suggestions concerning the symmetries of a potential notion of nfreeness.

Permanence of approximation properties for discrete quantum groups, Ann. Inst. Fourier 65 (2015), n^{o} 4, 14371467 : link and pdf (old version).
Abstract
We prove several results on the permanence of weak amenability and the Haagerup property for discrete quantum groups. In particular, we improve known facts on free products by allowing amalgamation over a finite quantum subgroup. We also define a notion of relative amenability for discrete quantum groups and link it with amenable equivalence of von Neumann algebras, giving additional permanence properties.

Fusion (semi)rings arising from quantum groups, J. Algebra 417 (2014), 161197 : link and pdf (revised version).
Abstract
We study the fusion rings arising from easy quantum groups. We classify all the possible free ones, answering a question of T. Banica and R. Vergnioux.
We then classify the possible groups of onedimensional representations for free easy quantum groups.
As an application, we give a unified proof of the Haagerup property for a broad class of easy quantum groups,
recovering as special cases previous results by M. Brannan and F. Lemeux.

On the representation theory of partition (easy) quantum groups (with M. Weber), J. Reine Angew. Math. 720 (2016), 155197 : link and pdf (old version).
Abstract
Compact matrix quantum groups are strongly determined by their intertwiner spaces,
due to a result by S.L. Woronowicz. In the case of easy quantum groups,
the intertwiner spaces are given by the combinatorics of partitions,
see the inital work of T. Banica and R. Speicher.
The philosophy is that all quantum algebraic properties of these objects should be visible in
their combinatorial data. We show that this is the case for their fusion rules
(i.e. for their representation theory). As a byproduct, we obtain a unified approach to the
fusion rules of the quantum permutation group S_{N}^{+},
the free orthogonal quantum group O_{N}^{+} as well as the
hyperoctahedral quantum group H_{N}^{+}.

Graphs of quantum groups and Kamenability (with P. Fima), Adv. Math. 260 (2014), 233280 : link and pdf (old version).
Abstract
Building on a construction of JP. Serre, we associate to any graph of C*algebras a maximal and a reduced fundamental C*algebra and use this theory to construct the fundamental quantum group of a graph of discrete quantum groups. To illustrate the properties of this construction, we then prove that if all the vertex quantum groups are amenable, the fundamental quantum group is Kamenable. This generalizes previous results of JulgValette, R. Vergnioux and P. Fima.

CCAP for universal discrete quantum groups (with K. De Commer and M. Yamashita, with an appendix by S. Vaes), Comm. Math. Phys. 331 (2014), n^{o} 2, 677701 : link and pdf (old version).
Abstract
In this paper we show that the discrete duals of the free orthogonal quantum groups have the Haagerup property and the completely contractive approximation property.
Analogous results also hold for the free unitary quantum groups and the quantum automorphism groups of finitedimensional C*algebras.
The proof relies on the monoidal equivalence between free orthogonal quantum groups and SU_{q}(2) quantum groups,
on the construction of a sufficient supply of bounded central functionals for SU_{q}(2) quantum groups,
and on the free product techniques of Ricard and Xu.
Our results generalize previous work in the Kac setting due to Brannan on the Haagerup property, and due to the second author on the CCAP.

Examples of weakly amenable discrete quantum groups, J. Funct. Anal. 265 (2013), n^{o} 9, 21642187 : link and pdf (old version).
Abstract
In this paper we give a polynomial bound for the completely bounded norm of the projections on coefficients of a fixed irreducible representation
in free orthogonal quantum groups. This enables us to compute their CowlingHaagerup constant,
which happens to be equal to 1. We then use an argument of monoidal equivalence to extend our result to other
free orthogonal quantum groups and quantum automorphism groups of finitedimensional C*algebras.
This gives in particular nonunimodular examples of weakly amenable discrete quantum groups which are not amenable.

A note on weak amenability for free products of discrete quantum groups, C. R. Acad. Sci. Paris 350 (2012), n^{o} 78, 403406 : link and pdf (old version).
Abstract
In this paper we prove that if two discrete quantum groups are weakly amenable with CowlingHaagerup constant equal to 1,
then their free product is also weakly amenable with CowlingHaagerup constant equal to 1.

Approximation properties for discrete quantum groups (PhD thesis) : pdf.
Abstract
This dissertation is concerned with the notion of approximation property for discrete quantum groups and in particular weak amenability.
Our goal is to apply techniques from geometric group theory to the study of quantum groups.
We first give a definition of weak amenability in the setting of discrete quantum groups and we develop some aspects of the general theory,
inspired by the classical case. We particularly focus on the notion of CowlingHaagerup constant.
We also define a notion of relative amenability in this context which allows us to prove an additional stability result.
Similar results are worked out for the Haagerup property. Eventually, we adress the question of free products of weakly amenable discrete quantum groups.
Using the work of E. Ricard and X. Qu on Kintchine inequalities for free products, we prove that if two discrete quantum groups have CowlingHaagerup constant equal to 1,
their free product again has CowlingHaagerup equal to 1.
Secondly, we give examples of weakly amenable discrete quantum groups.
To do this, we use the recent work of M. Brannan on the Haagerup property for free quantum groups together with ideas from various works on Haagerup inequalities.
More precisely, we give a polynomial bound for the norm of projections on coefficients of an irreducible representation of a
free orthogonal quantum groups which allows us to "cut off" M. Brannan's functions and compute the CowlingHaagerup constant.
Finally, we apply techniques of monoidal equivalence to extend these results to other classes of discrete quantum groups, some of which are not unimodular.