176,633 research outputs found
Precautionary Measures for Credit Risk Management in Jump Models
Sustaining efficiency and stability by properly controlling the equity to
asset ratio is one of the most important and difficult challenges in bank
management. Due to unexpected and abrupt decline of asset values, a bank must
closely monitor its net worth as well as market conditions, and one of its
important concerns is when to raise more capital so as not to violate capital
adequacy requirements. In this paper, we model the tradeoff between avoiding
costs of delay and premature capital raising, and solve the corresponding
optimal stopping problem. In order to model defaults in a bank's loan/credit
business portfolios, we represent its net worth by Levy processes, and solve
explicitly for the double exponential jump diffusion process and for a general
spectrally negative Levy process.Comment: 31 pages, 4 figure
The stellar IMF from Isothermal MHD Turbulence
We address the turbulent fragmentation scenario for the origin of the stellar
initial mass function (IMF), using a large set of numerical simulations of
randomly driven supersonic MHD turbulence. The turbulent fragmentation model
successfully predicts the main features of the observed stellar IMF assuming an
isothermal equation of state without any stellar feedback. As a test of the
model, we focus on the case of a magnetized isothermal gas, neglecting stellar
feedback, while pursuing a large dynamic range in both space and timescales
covering the full spectrum of stellar masses from brown dwarfs to massive
stars. Our simulations represent a generic 4 pc region within a typical
Galactic molecular cloud, with a mass of 3000 Msun and an rms velocity 10 times
the isothermal sound speed and 5 times the average Alfven velocity, in
agreement with observations. We achieve a maximum resolution of 50 au and a
maximum duration of star formation of 4.0 Myr, forming up to a thousand sink
particles whose mass distribution closely matches the observed stellar IMF. A
large set of medium-size simulations is used to test the sink particle
algorithm, while larger simulations are used to test the numerical convergence
of the IMF and the dependence of the IMF turnover on physical parameters
predicted by the turbulent fragmentation model. We find a clear trend toward
numerical convergence and strong support for the model predictions, including
the initial time evolution of the IMF. We conclude that the physics of
isothermal MHD turbulence is sufficient to explain the origin of the IMF.Comment: 25 pages, 21 figures, Accepted by Ap
Convergence of CI single center calculations of positron-atom interactions
The Configuration Interaction (CI) method using orbitals centered on the
nucleus has recently been applied to calculate the interactions of positrons
interacting with atoms. Computational investigations of the convergence
properties of binding energy, phase shift and annihilation rate with respect to
the maximum angular momentum of the orbital basis for the e^+Cu and PsH bound
states, and the e^+-H scattering system were completed. The annihilation rates
converge very slowly with angular momentum, and moreover the convergence with
radial basis dimension appears to be slower for high angular momentum. A number
of methods of completing the partial wave sum are compared, an approach based
on a Delta X_J = a/(J + 1/2)^n + b/(J + 1/2)^(n+1) form (with n = 4 for phase
shift (or energy) and n = 2 for the annihilation rate) seems to be preferred on
considerations of utility and underlying physical justification.Comment: 23 pages preprint RevTeX, 11 figures, submitted to PR
Extensions of the siesta dft code for simulation of molecules
We describe extensions to the siesta density functional theory (dft) code
[30], for the simulation of isolated molecules and their absorption spectra.
The extensions allow for: - Use of a multi-grid solver for the Poisson equation
on a finite dft mesh. Non-periodic, Dirichlet boundary conditions are computed
by expansion of the electric multipoles over spherical harmonics. - Truncation
of a molecular system by the method of design atom pseudo- potentials of Xiao
and Zhang[32]. - Electrostatic potential fitting to determine effective atomic
charges. - Derivation of electronic absorption transition energies and
oscillator stren- gths from the raw spectra produced by a recently described,
order O(N3), time-dependent dft code[21]. The code is furthermore integrated
within siesta as a post-processing option
Information Percolation with Equilibrium Search Dynamics
We solve for the equilibrium dynamics of information sharing in a large
population. Each agent is endowed with signals regarding the likely outcome of
a random variable of common concern. Individuals choose the effort with which
they search for others from whom they can gather additional information. When
two agents meet, they share their information. The information gathered is
further shared at subsequent meetings, and so on. Equilibria exist in which
agents search maximally until they acquire sufficient information precision,
and then minimally. A tax whose proceeds are used to subsidize the costs of
search improves information sharing and can in some cases increase welfare. On
the other hand, endowing agents with public signals reduces information sharing
and can in some cases decrease welfare
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