34,866 research outputs found
Renormalization of Drift and Diffusivity in Random Gradient Flows
We investigate the relationship between the effective diffusivity and
effective drift of a particle moving in a random medium. The velocity of the
particle combines a white noise diffusion process with a local drift term that
depends linearly on the gradient of a gaussian random field with homogeneous
statistics. The theoretical analysis is confirmed by numerical simulation. For
the purely isotropic case the simulation, which measures the effective drift
directly in a constant gradient background field, confirms the result
previously obtained theoretically, that the effective diffusivity and effective
drift are renormalized by the same factor from their local values. For this
isotropic case we provide an intuitive explanation, based on a {\it spatial}
average of local drift, for the renormalization of the effective drift
parameter relative to its local value. We also investigate situations in which
the isotropy is broken by the tensorial relationship of the local drift to the
gradient of the random field. We find that the numerical simulation confirms a
relatively simple renormalization group calculation for the effective
diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep
Metropolized Randomized Maximum Likelihood for sampling from multimodal distributions
This article describes a method for using optimization to derive efficient
independent transition functions for Markov chain Monte Carlo simulations. Our
interest is in sampling from a posterior density for problems in which
the dimension of the model space is large, is multimodal with regions
of low probability separating the modes, and evaluation of the likelihood is
expensive. We restrict our attention to the special case for which the target
density is the product of a multivariate Gaussian prior and a likelihood
function for which the errors in observations are additive and Gaussian
Thermal Casimir effect with soft boundary conditions
We consider the thermal Casimir effect in systems of parallel plates coupled
to a mass-less free field theory via quadratic interaction terms which suppress
(i) the field on the plates (ii) the gradient of the field in the plane of the
plates. These boundary interactions correspond to (i) the presence of an
electrolyte in the plates and (ii) a uniform field of dipoles, in the plates,
which are polarizable in the plane of the plates. These boundary interactions
lead to Robin type boundary conditions in the case where there is no field
outside the two plates. In the appropriate limit, in both cases Dirichlet
boundary conditions are obtained but we show that in case (i) the Dirichlet
limit breaks down at short inter-plate distances and in (ii) it breaks down at
large distances. The behavior of the two plate system is also seen to be highly
dependent on whether the system is open or closed. In addition we analyze the
Casimir force on a third plate placed between two outer plates. The force
acting on the central plate is shown to be highly sensitive to whether or not
the fluctuating scalar field is present in the region exterior to the two
confining plates.Comment: 8 pages RevTex, 2 .eps figure
Metastable states of spin glasses on random thin graphs
In this paper we calculate the mean number of metastable states for spin
glasses on so called random thin graphs with couplings taken from a symmetric
binary distribution . Thin graphs are graphs where the local
connectivity of each site is fixed to some value . As in totally connected
mean field models we find that the number of metastable states increases
exponentially with the system size. Furthermore we find that the average number
of metastable states decreases as in agreement with previous studies
showing that finite connectivity corrections of order increase the number
of metastable states with respect to the totally connected mean field limit. We
also prove that the average number of metastable states in the limit
is finite and converges to the average number of metastable states
in the Sherrington-Kirkpatrick model. An annealed calculation for the number of
metastable states of energy is also carried out giving a lower
bound on the ground state energy of these spin glasses. For small one may
obtain analytic expressions for .Comment: 13 pages LateX, 3 figures .ep
Dynamical transition for a particle in a squared Gaussian potential
We study the problem of a Brownian particle diffusing in finite dimensions in
a potential given by where is Gaussian random field.
Exact results for the diffusion constant in the high temperature phase are
given in one and two dimensions and it is shown to vanish in a power-law
fashion at the dynamical transition temperature. Our results are confronted
with numerical simulations where the Gaussian field is constructed, in a
standard way, as a sum over random Fourier modes. We show that when the number
of Fourier modes is finite the low temperature diffusion constant becomes
non-zero and has an Arrhenius form. Thus we have a simple model with a fully
understood finite size scaling theory for the dynamical transition. In addition
we analyse the nature of the anomalous diffusion in the low temperature regime
and show that the anomalous exponent agrees with that predicted by a trap
model.Comment: 18 pages, 4 figures .eps, JPA styl
Abundance of Rice Root Aphid Among Selected Plant Species and on Plants Grown With Different Soil-Surface Media
The rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), is distributed worldwide and colonizes a wide range of plants. However, relatively little is known about the suitability of different host plants, optimal rearing techniques, and the aphidâs impact on plant fitness. To improve understanding of these factors, laboratory experiments were conducted to compare the abundance of rice root aphid on plants grown using three different soil-surface media and among selected monocotyledonous and dicotyledonous plants. Rice root aphid was more abundant on plants grown with a sandy soil surface than a surface with fine wood chips or only bare non-sandy soil. Rice root aphid was more abundant on âElbonâ rye than on âBart 38,â âDart,â âFletcherâ and âRamona 50â wheat. More winged rice root aphids were produced on Elbon rye than on Dart wheat, but the number of winged aphids on Elbon rye did not differ from that on other wheat lines. Rice root aphid was more abundant on Elbon rye and âTAM 110â wheat than on âMarmin,â âMarshallâ and âSharpâ wheat. Additional observations with monocotyledonous plants showed that abundance of rice root aphid on âKivu 85â triticale was comparable to that on Elbon rye. Rice root aphid did not reproduce on potato or soybean, although winged adults persisted up to 24 days on caged potato plants. The implications of differential abundance of rice root aphid on plants are discussed in regard to colony rearing, future experiments and possible pest management considerations
Effective diffusion constant in a two dimensional medium of charged point scatterers
We obtain exact results for the effective diffusion constant of a two
dimensional Langevin tracer particle in the force field generated by charged
point scatterers with quenched positions. We show that if the point scatterers
have a screened Coulomb (Yukawa) potential and are uniformly and independently
distributed then the effective diffusion constant obeys the
Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained
for pure Coulomb scatterers frozen in an equilibrium configuration of the same
temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure
Using Experimental Economics to Measure Social Capital And Predict Financial Decisions
Questions remain as to whether results from experimental economics games are generalizable to real decisions in non-laboratory settings. Furthermore, important questions persist about whether social capital can help solve seemingly missing credit markets. I conduct two experiments, a Trust game and a Public Goods game, and a survey to measure social capital. I then examine whether behavior in the games predicts repayment of loans to a Peruvian group lending microfinance program. Since the structure of these loans relies heavily on social capital to enforce repayment, this is a relevant and important test of the games, as well as of other measures of social capital. I find that individuals identified as "trustworthy" by the Trust game are in fact less likely to default on their loans. I do not find similar support for the Trust game as a measure of trust.trust game, experimental economics, microfinance
Social Connections and Group Banking
Lending to the poor is expensive due to high screening, monitoring, and enforcement costs. Group lending advocates believe lenders overcome this by harnessing social connections. Using data from FINCA-Peru, I exploit a quasi-random group formation process to find evidence of peers successfully monitoring and enforcing joint-liability loans. Individuals with stronger social connections to their fellow group members (i.e., either living closer or being of a similar culture) have higher repayment and higher savings. Furthermore, I observe direct evidence that relationships deteriorate after default, and that through successful monitoring, individuals know who to punish and who not to punish after default.Microfinance, Group lending, informal savings, social capital
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