10,587 research outputs found
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
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 ( 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
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 -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
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
CCS and NH_3 Emission Associated with Low-Mass Young Stellar Objects
In this work we present a sensitive and systematic single-dish survey of CCS emission (complemented with ammonia observations) at 1 cm, toward a sample of low- and intermediate-mass young star-forming regions known to harbor water maser emission, made with NASA's 70 m antenna at Robledo de Chavela, Spain. Out of the 40 star-forming regions surveyed in the CCS (2_(1)-1_(0)) line, only six low-mass sources show CCS emission: one transitional object between the prestellar and protostellar Class 0 phase (GF9-2), three Class 0 protostars (L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a young T Tauri star (NGC 2071 North). Since CCS is considered an "early-time" (≲10^5 yr) molecule, we explain these results by either proposing a revision of the classification of the age of NGC 2071 North and L1251A, or suggesting the possibility that the particular physical conditions and processes of each source affect the destruction/production of the CCS. No statistically significant relationship was found between the presence of CCS and parameters of the molecular outflows and their driving sources. Nevertheless, we found a significant relationship between the detectability of CCS and the ammonia peak intensity (higher in regions with CCS), but not with its integrated intensity. This tendency may suggest that the narrower ammonia line widths in the less turbulent medium associated with younger cores may compensate for the differences in ammonia peak intensity, rendering differences in integrated intensity negligible. From the CCS detection rate we derive a lifetime of this molecule of ≃(0.7-3) × 10^4 yr in low-mass star-forming regions
No Hubble Bubble in the Local Universe
Zehavi et al. (1998) have suggested that the Hubble flow within 70/h Mpc may
be accelerated by the existence of a void centered on the Local Group. Its
underdensity would be ~20 %, which would result in a local Hubble distortion of
about 6.5 %. We have combined the peculiar velocity data of two samples of
clusters of galaxies, SCI and SCII, to investigate the amplitude of Hubble
distortions to 200/h Mpc. Our results are not supportive of that conclusion.
The amplitude of a possible distortion in the Hubble flow within 70/h Mpc in
the SCI+SCII merged data is 0.010\pm0.022. The largest, and still quite
marginal, geocentric deviation from smooth Hubble flow consistent with that
data set is a shell with (Delta H)/H =0.027\pm0.023, centered at hd = 101 Mpc
and extending over some 30/h Mpc. Our results are thus consistent with a Hubble
flow that, on distances in excess of about 50/h Mpc, is remarkably smooth.Comment: 11 pages, 1 tables, 1 figure; uses AAS LaTex; to appear in ApJ Nov 9
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
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
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