The Chemical Evolution of the Milky Way: the Three Infall Model

We present a new chemical evolution model for the Galaxy that assumes three main infall episodes of primordial gas for the formation of halo, thick and thin disk, respectively. We compare our results with selected data taking into account NLTE effects. The most important parameters of the model are (i) the timescale for gas accretion, (ii) the efficiency of star formation and (iii) a threshold in the gas density for the star formation process, for each Galactic component. We find that, in order to best fit the features of the solar neighbourhood, the halo and thick disk must form on short timescales (~0.2 and ~1.25 Gyr, respectively), while a longer timescale is required for the thin-disk formation. The efficiency of star formation must be maximum (10 Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk formation. Also the threshold gas density for star formation is suggested to be different in the three Galactic components. Our main conclusion is that in the framework of our model an independent episode of accretion of extragalactic gas, which gives rise to a burst of star formation, is fundamental to explain the formation of the thick disk. We discuss our results in comparison to previous studies and in the framework of modern galaxy formation theories.Comment: 12 pages, 7 figures, accepted for publication in MNRA

Knightian Auctions

We study single-good auctions in a setting where each player knows his own valuation only within a constant multiplicative factor \delta{} in (0,1), and the mechanism designer knows \delta. The classical notions of implementation in dominant strategies and implementation in undominated strategies are naturally extended to this setting, but their power is vastly different. On the negative side, we prove that no dominant-strategy mechanism can guarantee social welfare that is significantly better than that achievable by assigning the good to a random player. On the positive side, we provide tight upper and lower bounds for the fraction of the maximum social welfare achievable in undominated strategies, whether deterministically or probabilistically

Knightian Analysis of the Vickrey Mechanism

We analyze the Vickrey mechanism for auctions of multiple identical goods when the players have both Knightian uncertainty over their own valuations and incomplete preferences. In this model, the Vickrey mechanism is no longer dominant-strategy, and we prove that all dominant-strategy mechanisms are inadequate. However, we also prove that, in undominated strategies, the social welfare produced by the Vickrey mechanism in the worst case is not only very good, but also essentially optimal.Comment: To appear in Econometric

Perfect Implementation

Privacy and trust aect our strategic thinking, yet they have not been precisely modeled in mechanism design. In settings of incomplete information, traditional implementations of a normal-form mechanism - by disregarding the players' privacy, or assuming trust in a mediator - may fail to reach the mechanism's objectives. We thus investigate implementations of a new type. We put forward the notion of a perfect implementation of a normal-form mechanism M: in essence, a concrete extensive-form mechanism exactly preserving all strategic properties of M, without relying on a trusted mediator or violating the privacy of the players. We prove that any normal-form mechanism can be perfectly implemented by a verifiable mediator using envelopes and an envelope-randomizing device (i.e., the same tools used for running fair lotteries or tallying secret votes). Differently from a trusted mediator, a veriable one only performs prescribed public actions, so that everyone can verify that he is acting properly, and that he never learns any information that should remain private

Networked media actions as hacktions: rethinking resistance(s) in media ecologies

This article attempts to rethink a notion of resistance for contemporary forms of dissent and opposition that are increasingly organised through digital media and networks. Applying a post-human compass on hacking, a processual reading of the hack is implied to propose a movement towards the idea of hacktions. Hacktions are networked media actions that involve an aesthetic register of de-subjective creativity, aiming towards systematic disruptions: the active resistances of a media ecological dysfunctionality

Collusion-Resilient Revenue In Combinatorial Auctions

In auctions of a single good, the second-price mechanism achieves, in dominantstrategies, a revenue benchmark that is naturally high and resilient to anypossible collusion.We show how to achieve, to the maximum extent possible, the same propertiesin combinatorial auctions

Very Simple and Efficient Byzantine Agreement

We present a very simple, cryptographic, binary Byzantine-agreement protocol that, with n >= 3t+1 >= 3 players, at most t of which are malicious, halts in expected 9 rounds