Optimal Fragile Financial Networks

We study a financial network characterized by the presence of depositors, banks and their shareholders. Belonging to a financial network is beneficial for both the depositors and banks' shareholders since the return to investment increases with the number of banks connected. However, the network is fragile since banks, which invest on behalf of the depositors, can gamble with depositors' money (making an investment that is dominated in expected terms) when not sufficiently capitalized. The bankruptcy of a bank negatively affects the banks connected to it in the network. First, we compute the social planner solution and the efficient financial network is characterized by a core-periphery structure. Second, we analyze the decentralized solution showing under which conditions participating in a fragile financial network is ex-ante optimal. In particular, we show that this is optimal when the probability of bankruptcy is sufficiently low giving rationale of financial fragility as a rare phenomenon. Finally, we analyze the efficiency of the decentralized financial network. Again, if the probability of bankruptcy is sufficiently low the structure of the decentralized financial network is equal to the e¢ cient one, yielding an ex- pected payo¤ arbitrarily close to the efficient one. However, the investment decision is not the same. That is, in the decentralized network some banks will gamble as compared to the socially preferred outcome.Financial Network;Moral Hazard;Financial Fragility

Neutrino propagation in Neutron Matter and the Nuclear Equation of State

We study the propagation of neutrinos inside dense matter under the conditions prevailing in a proto-neutron star. Equations of state obtained with different nuclear effective interactions (Skyrme type and Gogny type) are first discussed. It is found that for many interactions, spin and/or isospin instabilities occur at densities larger than the saturation density of nuclear matter. From this study we select two representative interactions, SLy230b and D1P. We calculate the response functions in pure neutron matter where nuclear correlations are described at the Hartree-Fock plus RPA level. These response functions allow us to evaluate neutrino mean free paths corresponding to neutral current processes.Comment: 8 pages, 7 figures, to appear in "The Nuclear Many-Body Problem 2001", NATO Science Series II (Kluwer Academic Publishers

Neutrino mean free path and in-medium nuclear interaction

Neutrinos produced during the collapse of a massive star are trapped in a nuclear medium (the proto-neutron star). Typically, neutrino energies (10-100 MeV) are of the order of nuclear giant resonances energies. Hence, neutrino propagation is modified by the possibility of coherent scattering on nucleons. We have compared the predictions of different nuclear interaction models. It turns out that their main discrepancies are related to the density dependence of the k-effective mass as well as to the onset of instabilities as density increases. This last point had led us to a systematic study of instabilities of infinite matter with effective Skyrme-type interactions. We have shown that for such interactions there is always a critical density, above which the system becomes unstable.Comment: 4 pages, 4 figures, Proceedings of the 17th Divisional Conference on Nuclear Physics in Astrophysics (NPDC17), 30th September - 4th October 2002, ATOMKI, Debrecen, Hungary, to appear in Nuclear Physics

Fair allocation in evolving networks

We consider networks evolving over time within an infinite-horizon dynamic setting. Transitions from one network to another are given by a stationary transition probability matrix. We study the problem of fairly and efficiently allocating the value of a network at any point in time among its participants, assuming that agents discount the future by some common discount factor #. An allocation rule is called component efficient if it distributes the total value of a connected network among its participants and it is called expected fair if for every direct connection both participants expect to loose or gain the same amount in the future from breaking this connection at time zero. Our main result is that for every transition probability matrix and for almost every # there exists a unique allocation rule which is component e#cient and expected fair. We provide a formula to compute this allocation rule. In general, this allocation rule is different from a stage-wise application of the Myerson value. We also provide a sufficient condition on the transition probability matrix such that the component efficient and expected fair allocation rule is equal to the Myerson value

Is a Simple Collisionless Relic Dark Matter Particle Ruled Out?

The central densities of dark matter (DM) halos are much lower than predicted in cold DM models of structure formation. Confirmation that they have cores with a finite central density would allow us to rule out many popular types of collisionless particle as candidates for DM. Any model that leads to cusped halos (such as cold DM) is already facing serious difficulties on small scales and hot DM models have been excluded. Here I show that fermionic warm DM is inconsistent with the wide range of phase space densities in the DM halos of well-observed nearby galaxies.Comment: 6 pages, 1 figure, LaTeX uses emulateapj.sty, revised version to appear in ApJ Letters. Argument clarified and strengthened in response to criticism, conclusions little change

Supermassive Black Holes and Galaxy Formation

The formation of supermassive black holes (SMBH) is intimately related to galaxy formation, although precisely how remains a mystery. I speculate that formation of, and feedback from, SMBH may alleviate problems that have arisen in our understanding of the cores of dark halos of galaxies.Comment: Talk at conference on Matter in the Universe, March 2001, ISSI Ber

From Theory to Practice: Plug and Play with Succinct Data Structures

Engineering efficient implementations of compact and succinct structures is a time-consuming and challenging task, since there is no standard library of easy-to- use, highly optimized, and composable components. One consequence is that measuring the practical impact of new theoretical proposals is a difficult task, since older base- line implementations may not rely on the same basic components, and reimplementing from scratch can be very time-consuming. In this paper we present a framework for experimentation with succinct data structures, providing a large set of configurable components, together with tests, benchmarks, and tools to analyze resource requirements. We demonstrate the functionality of the framework by recomposing succinct solutions for document retrieval.Comment: 10 pages, 4 figures, 3 table

La repoblació del conill de bosc incrementa la sarna

Les malalties víriques, la caça intensiva i les noves pràctiques agrícoles han comportat que el conill de bosc de la Península Ibèrica sigui una espècie "gairebé amenaçada". Però, segons un estudi d'investigadors del Departament de Medicina i Cirurgia Animal de la UAB, la repoblació de conills que es duu a terme a molts llocs de la Península introdueix noves malalties en les poblacions receptores, com la sarna sarcòptica que perfora l'epidermis de més de cent espècies de mamífers, inclòs l'home. A la vista d'aquests resultats, els investigadors recomanen un estricte control sanitari dels conills alliberats.Las enfermedades víricas, la caza intensiva y las nuevas prácticas agrícolas han supuesto que el conejo de monte de la Península Ibérica sea una especie "casi amenazada". Pero, según un estudio de investigadores del Departamento de Medicina y Cirugía Animal de la UAB, la repoblación de conejos que se lleva a cabo en muchos lugares de la Península introduce nuevas enfermedades en las poblaciones receptoras, como la sarna sarcóptica que perfora la epidermis de más de cien especies de mamíferos, incluído el hombre. A la vista de estos resultados, los investigadores recomiendan un estricto control sanitario de los conejos liberados

Axially deformed solution of the Skyrme-Hartree-Fock-Bogolyubov equations using the transformed harmonic oscillator basis (III) hfbtho (v3.00): a new version of the program

We describe the new version 3.00 of the code HFBTHO that solves the nuclear Hartree-Fock (HF) or Hartree-Fock-Bogolyubov (HFB) problem by using the cylindrical transformed deformed harmonic oscillator basis. In the new version, we have implemented the following features: (i) the full Gogny force in both particle-hole and particle-particle channels, (ii) the calculation of the nuclear collective inertia at the perturbative cranking approximation, (iii) the calculation of fission fragment charge, mass and deformations based on the determination of the neck (iv) the regularization of zero-range pairing forces (v) the calculation of localization functions (vi)MPI interface for large-scale mass table calculations.Comment: 29 pages, 3 figures, 4 tables; Submitted to Computer Physics Communication