Insurance and Incentives in Sharecropping

This essay surveys some recent empirical works about sharecropping. The basic theoretical trade-offs are discussed in the introduction. Section 1 discusses the empirical research on resource allocation. This section is divided in two subsections: one studying the effects of tenure stability on land improvements, and another comparing the impact of different share rates on input use and farm productivity. Section 2 surveys works testing different arguments raised to explain the design of tenancy contracts. The essay then concludes with a brief summary discussing some important policy implications.

The Origin of Radio Emission in Low-Luminosity Active Galactic Nuclei: Jets, Accretion Flows, or Both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of $10^3-10^4$ times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures ($> 10^9$ K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared to the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.Comment: To appear in ApJ (Letters). 4 page

Hurst Coefficient in long time series of population size: Model for two plant populations with different reproductive strategies

Can the fractal dimension of fluctuations in population size be used to estimate extinction risk? The problem with estimating this fractal dimension is that the lengths of the time series are usually too short for conclusive results. This study answered this question with long time series data obtained from an iterative competition model. This model produces competitive extinction at different perturbation intensities for two different germination strategies: germination of all seeds vs. dormancy in half the seeds. This provided long time series of 900 years and different extinction risks. The results support the hypothesis for the effectiveness of the Hurst coefficient for estimating extinction risk

Insurance and Incentives in Sharecropping

This essay surveys some recent empirical works about sharecropping. The basic theoretical trade-offs are discussed in the introduction. Section 1 discusses the empirical research on resource allocation. This section is divided in two subsections: one studying the effects of tenure stability on land improvements, and another comparing the impact of different share rates on input use and farm productivity. Section 2 surveys works testing different arguments raised to explain the design of tenancy contracts. The essay then concludes with a brief summary discussing some important policy implications

3D simulations of Einstein's equations: symmetric hyperbolicity, live gauges and dynamic control of the constraints

We present three-dimensional simulations of Einstein equations implementing a symmetric hyperbolic system of equations with dynamical lapse. The numerical implementation makes use of techniques that guarantee linear numerical stability for the associated initial-boundary value problem. The code is first tested with a gauge wave solution, where rather larger amplitudes and for significantly longer times are obtained with respect to other state of the art implementations. Additionally, by minimizing a suitably defined energy for the constraints in terms of free constraint-functions in the formulation one can dynamically single out preferred values of these functions for the problem at hand. We apply the technique to fully three-dimensional simulations of a stationary black hole spacetime with excision of the singularity, considerably extending the lifetime of the simulations.Comment: 21 pages. To appear in PR

A Radio Polarimetric Study of the Galactic Center Threads

Multi-frequency, polarimetric VLA observations of the non-thermal filaments (NTF's), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads are presented at 20, 6, 3.6 and 2 cm, with high enough spatial resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTF's. With resolutions as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and little substructure internal to the filament. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, alpha=-0.5 between 90 and 6 cm, and becomes much steeper (alpha=-2.0) between 6 and 2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime, or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at 6 and 3.6 cm confirm the non-thermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70% in some regions, although the polarized emission is patchy. Large rotation measures (RM > 2000 rad/m2) have been observed with irregular variations across the filament.The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis.Comment: 19 pages, incl. 24 figs; to appear in the Astrophysical Journa

A possible origin of superconducting currents in cosmic strings

The scattering and capture of right-handed neutrinos by an Abelian cosmic string in the SO(10) grand unification model are considered. The scattering cross-section of neutrinos per unit length due to the interaction with the gauge and Higgs fields of the string is much larger in its scaling regime than in the friction one because of the larger infrared cutoff of the former.The probability of capture in a zero mode of the string accompanied by the emission of a gauge or Higgs boson shows a resonant peak for neutrino momentum of the order of its mass. Considering the decrease of number of strings per unit comoving volume in the scaling epoch the cosmological consequences of the superconducting strings formed in this regime will be much smaller than those which could be produced already in the friction one.Comment: 14 pages Latex, 4 figues/ep

Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6

Few examples of magnetic systems displaying a transition to pure dipolar magnetic order are known to date, and single-molecule magnets can provide an interesting example. The molecular cluster spins and thus their dipolar interaction energy can be quite high, leading to reasonably accessible ordering temperatures, provided the crystal field anisotropy is sufficiently small. This condition can be met for molecular clusters of sufficiently high symmetry, as for the Mn6 compound studied here. Magnetic specific heat and susceptibility experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K. Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and account for the correct value of T_{c}. In high magnetic fields we detected the contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic timescale of nuclear relaxation. This was compared with results obtained directly from pulse-NMR experiments. The data are in good mutual agreement and can be well described by the theory for magnetic relaxation in highly polarized paramagnetic crystals and for dynamic nuclear polarization, which we extensively review. The experiments provide an interesting comparison with the recently investigated nuclear spin dynamics in the anisotropic single molecule magnet Mn12-ac.Comment: 19 pages, 11 eps figures. Contains extensive discussions on dipolar ordering, specific heat and nuclear relaxation in molecular magnet

On the smoothness of nonlinear system identification

We shed new light on the \textit{smoothness} of optimization problems arising in prediction error parameter estimation of linear and nonlinear systems. We show that for regions of the parameter space where the model is not contractive, the Lipschitz constant and $\beta$-smoothness of the objective function might blow up exponentially with the simulation length, making it hard to numerically find minima within those regions or, even, to escape from them. In addition to providing theoretical understanding of this problem, this paper also proposes the use of multiple shooting as a viable solution. The proposed method minimizes the error between a prediction model and the observed values. Rather than running the prediction model over the entire dataset, multiple shooting splits the data into smaller subsets and runs the prediction model over each subset, making the simulation length a design parameter and making it possible to solve problems that would be infeasible using a standard approach. The equivalence to the original problem is obtained by including constraints in the optimization. The new method is illustrated by estimating the parameters of nonlinear systems with chaotic or unstable behavior, as well as neural networks. We also present a comparative analysis of the proposed method with multi-step-ahead prediction error minimization

General Equilibrium with Endogenous Securities and Moral Hazard

This paper studies a class of general equilibrium economies in which the individuals' endowments depend on privately observed effort choices and the financial markets are endogenous. The environment is modeled as a two-stage game. Individuals first make strategic financial-innovation decisions. They then act in a Radner-type economy with the previously designed securities. Consumption goods, portfolios, and effort levels are chosen competitively (i.e., taking prices as given). An equilibrium concept is adapted for these moral hazard economies and its existence is proven. It is shown through an example how incentive motives might lead to the endogenous emergence of financial incompleteness.general equilibrium, moral hazard, endogenous incomplete markets, non-exclusive securities