A four moments theorem for Gamma limits on a Poisson chaos

This paper deals with sequences of random variables belonging to a fixed chaos of order q generated by a Poisson random measure on a Polish space. The problem is investigated whether convergence of the third and fourth moment of such a suitably normalized sequence to the third and fourth moment of a centred Gamma law implies convergence in distribution of the involved random variables. A positive answer is obtained for q = 2 and q = 4. The proof of this four moments theorem is based on a number of new estimates for contraction norms. Applications concern homogeneous sums and U-statistics on the Poisson space.

Eluding oblivion with smart stochastic selection of synaptic updates

The variables involved in the equations that describe realistic synaptic dynamics always vary in a limited range. Their boundedness makes the synapses forgetful, not for the mere passage of time, but because new experiences overwrite old memories. The forgetting rate depends on how many synapses are modified by each new experience: many changes means fast learning and fast forgetting, whereas few changes means slow learning and long memory retention. Reducing the average number of modified synapses can extend the memory span at the price of a reduced amount of information stored when a new experience is memorized. Every trick which allows to slow down the learning process in a smart way can improve the memory performance. We review some of the tricks that allow to elude fast forgetting (oblivion). They are based on the stochastic selection of the synapses whose modifications are actually consolidated following each new experience. In practice only a randomly selected, small fraction of the synapses eligible for an update are actually modified. This allows to acquire the amount of information necessary to retrieve the memory without compromising the retention of old experiences. The fraction of modified synapses can be further reduced in a smart way by changing synapses only when it is really necessary, i.e. when the post-synaptic neuron does not respond as desired. Finally we show that such a stochastic selection emerges naturally from spike driven synaptic dynamics which read noisy pre and post-synaptic neural activities. These activities can actually be generated by a chaotic system.

Relating in situ gas measurements to the surface outgassing properties of cometary nuclei

The sensitivity of the gas flow field to changes in different initial conditions has been studied for the case of a highly simplified cometary nucleus model. The nucleus model simulated a homogeneously outgassing sphere with a more active ring around an axis of symmetry. The varied initial conditions were the number density of the homogeneous region, the surface temperature, and the composition of the flow (varying amounts of H2O and CO2) from the active ring. The sensitivity analysis was performed using the Polynomial Chaos Expansion (PCE) method. Direct Simulation Monte Carlo (DSMC) was used for the flow, thereby allowing strong deviations from local thermal equilibrium. The PCE approach can be used to produce a sensitivity analysis with only four runs per modified input parameter and allows one to study and quantify non-linear responses of measurable parameters to linear changes in the input over a wide range. Hence the PCE allows one to obtain a functional relationship between the flow field properties at every point in the inner coma and the input conditions. It is for example shown that the velocity and the temperature of the background gas are not simply linear functions of the initial number density at the source. As probably expected, the main influence on the resulting flow field parameter is the corresponding initial parameter (i.e. the initial number density determines the background number density, the temperature of the surface determines the flow field temperature, etc.). However, the velocity of the flow field is also influenced by the surface temperature while the number density is not sensitive to the surface temperature at all in our model set-up. Another example is the change in the composition of the flow over the active area. Such changes can be seen in the velocity but again not in the number density. Although this study uses only a simple test case, we suggest that the approach, when applied to a real case in 3D, should assist in identifying the sensitivity of gas parameters measured in situ by, for example, the Rosetta spacecraft to the surface boundary conditions and vice versa.

Тоска по домашнему очагу: Мандельштам, архитектура и русский язык

Le poète Ossip Mandelstam a été habité toute sa vie par un sentiment d’exil et de déracinement dont les causes sont diverses (« chaos judaïque » de son enfance, déménagements incessants, expérience de catastrophes historiques, etc.) : son ralliement au mouvement acméiste peut se comprendre comme une tentative de lutter contre ce sentiment au niveau de l’imaginaire. Le programme acméiste, contrairement au mouvement symboliste qui déprécie les choses d’ici bas au profit d’une réalité transcendante, peut en effet se définir comme une tentative de rendre au monde une certaine matérialité et hospitalité. Dans ce contexte, Mandelstam privilégie le motif architectural, qui lui fournit de nombreuses images de l’abri, du logement et de la protection, tout particulièrement dans son recueil intitulé Pierre. Néanmoins, si les images architecturales convoquées par Mandelstam apaisent momentanément sa soif d’un foyer, elles ne sont pas suffisantes : le poète invoque encore le pouvoir et le statut ontologique particuliers de la langue russe, seule langue capable selon lui de retrouver l’âme des choses et de faire que le poète se sente chez soi dans le monde.

Die Ordnung des Theaters. Eine Soziologie der Regie

Die Theaterregie ist heute so entfesselt, der künstlerische Möglichkeitsraum so offen wie nie. Kreatives Chaos allenthalben? Nicht wirklich. Denis Hänzi untersucht, wie aus dem Wechselspiel von individuellen Arbeitsweisen und institutionellen Arrangements, alten Idealen und neuen Realitäten das Gefüge der deutschsprachigen Theaterlandschaft hervorgeht. Mit einer charismatheoretischen Erweiterung des Bourdieu'schen Habitus/Feld-Konzepts wird ein innovativer Ansatz zur Analyse gegenwärtiger Kunstwelten formuliert. Dass die »Ordnung des Theaters« zusehends Züge einer Erfolgskultur trägt, welche die männliche Dominanz im Regieberuf zu verfestigen scheint, ist einer von vielen erhellenden Befunden, die das Buch auch für Theaterschaffende und -interessierte spannend machen.