Stability of spherically symmetric steady states in galactic dynamics against general perturbations

Certain steady states of the Vlasov-Poisson system can be characterized as minimizers of an energy-Casimir functional, and this fact implies a nonlinear stability property of such steady states. In previous investigations by Y. Guo and the author stability was obtained only with respect to spherically symmetric perturbations. In the present investigation we show how to remove this unphysical restriction.Comment: 19 pages LaTe

Dark matter in massive galaxies

The spatial distributions of luminous and dark matter in massive early-type galaxies reflect the formation processes which shaped these systems. This article reviews the predictions of cosmological simulations for the dark and baryonic components of ETGs, and the observational constraints from lensing, hydrostatic X-ray gas athmospheres, and outer halo stellar dynamics.Comment: Invited review, 10 pages, 1 figure, LaTeX. To appear in Proc. of IAU Symposium 295 held in Beijing, China, August 2012, eds. D. Thomas, A. Pasquali and I. Ferreras, Cambridge Univ. Pres

Gradualism vs Cold Turkey : how to establish credibility for the ECB

The paper analyzes the incentive for the ECB to establish reputation by pursuing a restrictive policy right at the start of its operation. The bank is modelled as risk averse with respect to deviations of both inflation and output from her target. The public, being imperfectly informed about the bank’s preferences uses observed inflation as (imperfect) signal for the unknown preferences. Under linear learning rules - which are commonly used in the literature - a gradual build up of reputation is the optimal response. The paper shows that such a linear learning rule is not consistent with efficient signaling. It is shown that in a game with efficient signaling, a cold turkey approach - allowing for deflation - is optimal for a strong bank - accepting high current output losses at the beginning in order to demonstrate its toughness. JEL classification: D 82, E 58Die Arbeit untersucht die Anreize der Europäischen Zentralbank, in der Startphase durch restriktive Politik Reputation aufzubauen. Die Öffentlichkeit kennt die Präferenzen der Zentralbank nicht; sie verwendet die beobachtete Inflationsrate als (imperfektes) Signal. Wird eine lineare Lernregel unterstellt - der Standardfall in der Literatur - erweist es sich als optimal, hohe Inflationserwartungen zumindest teilweise zu akkommodieren und so Reputation nur schrittweise aufzubauen. Die Arbeit zeigt aber, daß eine solche lineare Lernregel mit effizientem Signalverhalten nicht konsistent ist. Bei effizientem Signalisieren kann es für eine harte Zentralbank optimal sein, in der Startphase durch eine sehr restriktive, deflationäre Politik ihre Präferenzen zu offenbaren. JEL classification: D 82, E 5

Theoretical Update on Rare K Decays

We review the status of rare kaon decays, concentrating on modes with sensitivity to short-distance flavour physics.Comment: Invited talk presented at Daphne 99, Frascati, Nov. 1999, 13 pages, 3 figure