# Posters

This is the list of posters presented at the posters session that will take place during the school on Monday, July 17, 2017 (See Schedule). The posters will be displayed during all the school.

Posters can be printed in A2 format (easy to bring with you). No on-site printing service is available.

Click on “abstract” for displaying and hiding individual abstracts.

This poster is based on joint work with A. Sitarz [1]. For a complex parameter \( q \), the algebra \( \mathcal{A}(SU_q(2)) \) was shown to be a braided Hopf algebra in some braided tensor category [2]. Our original motivation was to construct braided differential calculi on this algebra. We show that there is a class of Hopf algebras for which, using the general framework of twisting, one can construct their braided versions. In particular, the Hopf algebras of \( SU_q(n) \) and the quantum double torus belong to this class. Furthermore, the twisting can also be applied to the (co-,bi-)modules over the braidable Hopf algebras, and permits us to consider braided differential calculi.

[1] A. Bochniak, A. Sitarz. *Braided Hopf algebras from twisting*. arXiv:1701.01106.

[2] P. Kasprzak, R. Meyer, S. Roy, S.L. Woronowicz. *Braided quantum \( SU(2) \) groups*, J. Noncommut. Geom. 10, p. 1161-1125 (2016)

In this poster we shall present a detailed account on the pseudodifferential calculus on noncommutative tori [1]. This calculus turns out to be very useful (see, e.g., [2]), but its analytic aspect hasn't been studied well in any literature. The aim of this poster is to fulfill this deficiency. This is joint work with Hyun-su Ha and Raphaël Ponge.

[1] A. Connes, *C*-algèbres et géométrie différentielle*. C.R. Acad. Sc. Paris, t. 290, Série A, 599-604, 1980.

[2] A. Connes, P. Tretkoff, *The Gauss-Bonnet theorem for the noncommutative two torus. Noncommutative geometry, arithmetic, and related topics*, pp. 141–158, Johns Hopkins Univ. Press, Baltimore, MD, 2011.

Einstein's causality is one of the fundamental principles underlying modern physical theories. Whereas it is readily implemented in classical physics founded on Lorentzian geometry, its status in quantum theory has long been controversial. On the mathematical side, the classical causal structure is a certain binary relation on the set events, i.e. points of spacetime. But in the quantum world the very notion of a point-like event becomes blurred and it is widely believed that the causal structure would also be afflicted with uncertainty.

However, together with my colleagues M. Eckstein and N. Franco, we have developed a rigorous framework, in which the causal structure remains rigid, although the events themselves become nonlocal. The idea, founded on the operational viewpoint on physics compelling us to exchange the classical spacetime for the space of states of some abstract algebra of observables, is incarnated with a Lorentzian version of noncommutative geometry à la Connes. The explanation of the basic concepts of our formalism will be followed by an illustrative example of an almost-commutative spacetime, which, moreover, sheds a new light on the peculiar properties of massive fermions.

[1] N. Franco and M. Eckstein, An algebraic formulation of causality for noncommutative geometry. *Classical Quant Grav*, 30(13):135007, 2013.

[2] M. Eckstein and T. Miller, Causality for nonlocal phenomena. *Ann. Henri Poincaré*. Published online 13 March 2017.

[3] M. Eckstein, N. Franco, and T. Miller, Noncommutative geometry of Zitterbewegung. *Phys Rev D*, 95:061701, 2017.

Vilenkin groups are a natural generalisation of the Walsh group, and have been extensively studied in the context of classical harmonic analysis. We consider the Vilenkin-Fourier transformation acting upon noncommutative \( L^p \) spaces associated to the hyperfinite type \( II_1 \) factor \( R \), and study weak type inequalities for the partial summation operators.

The structure of the Vilenkin groups induces a natural filtration on the factor \( R \), and so we also investigate noncommutative martingale spaces corresponding to this filtration.