Optimal search strategies of space-time coupled random walkers with finite lifetimes

We present a simple paradigm for detection of an immobile target by a space-time coupled random walker with a finite lifetime. The motion of the walker is characterized by linear displacements at a fixed speed and exponentially distributed duration, interrupted by random changes in the direction of motion and resumption of motion in the new direction with the same speed. We call these walkers "mortal creepers". A mortal creeper may die at any time during its motion according to an exponential decay law characterized by a finite mean death rate $\omega_m$. While still alive, the creeper has a finite mean frequency $\omega$ of change of the direction of motion. In particular, we consider the efficiency of the target search process, characterized by the probability that the creeper will eventually detect the target. Analytic results confirmed by numerical results show that there is an $\omega_m$-dependent optimal frequency $\omega=\omega_{opt}$ that maximizes the probability of eventual target detection. We work primarily in one-dimensional ($d=1$) domains and examine the role of initial conditions and of finite domain sizes. Numerical results in $d=2$ domains confirm the existence of an optimal frequency of change of direction, thereby suggesting that the observed effects are robust to changes in dimensionality. In the $d=1$ case, explicit expressions for the probability of target detection in the long time limit are given. In the case of an infinite domain, we compute the detection probability for arbitrary times and study its early- and late-time behavior. We further consider the survival probability of the target in the presence of many independent creepers beginning their motion at the same location and at the same time. We also consider a version of the standard "target problem" in which many creepers start at random locations at the same time.Comment: 18 pages, 7 figures. The title has been changed with respect to the one in the previous versio

Farmer perceptions of climate change risk and associated on-farm management strategies in Vermont, northeastern United States

Little research has been conducted on how agricultural producers in the northeastern United States conceptualize climate-related risk and how these farmers address risk through on-farm management strategies. Two years following Tropical Storm Irene, our team interviewed 15 farmers in order to investigate their perceptions of climate-related risk and how their decision-making was influenced by these perceptions. Our results show that Vermont farmers are concerned with both ecological and economic risk. Subthemes that emerged included geographic, topographic, and hydrological characteristics of farm sites; stability of land tenure; hydrological erosion; pest and disease pressure; market access; household financial stability; and floods. Farmers in our study believed that these risks are not new but that they are significantly intensified by climate change. Farmer responses were heavily focused on adaptation activities, with discussion of climate change mitigation activities notably absent. Psychological distance construal theory and hyperbolic discounting emerged as well-suited frames to explain why farmers reported adaptation activities but not mitigation strategies. Farmers will probably experience an increasing severity of climate-related impacts in the northeast region; therefore, information about climate-related risks coming from farmers\u27 personal experience should be integrated with forecasting data to help farmers plan effective adaptation strategies

Componentes minoritarios de aceites obtenidos de frutos de palmeras de la región amazónica

This study deals with the characterization of minor compounds in oils obtained from the mesocarp of fruits of the main palm species from the State of Amapá, Brazil, i.e. bacaba (Oenocarpus bacaba), buriti (Mauritia flexuosa), inajá (Maximiliana maripa), pupunha (Bactris gasipaes) and tucumã (Astrocaryum vulgare). The concentration of minor glyceridic compounds, i.e. dimeric triacylglycerols (TAG), the oxidized TAG and diacylglycerols (DAG) related to oil quality, and the compounds of unsaponifiable matter, i.e. hydrocarbons, aliphatic alcohols, sterols and tocopherols have been determined. The results indicate that the extracted oils had good initial quality, with DAG as the major glyceridic compound. The contents of hydrocarbons (50-734 mg·kg–1) and aliphatic alcohols (80-490 mg·kg–1) were highly variable with inajá oil containing the highest contents. In the case of tocopherols, buriti (1567 mg·kg–1) and tucumã (483 mg·kg–1) oils had the highest contents and the presence of significant amounts of tocotrienols was only detected in inajá oil. Finally, high concentrations of sterols were found in all the samples, particularly in the oils from pupunha (4456 mg·kg–1) and tucumã (2708 mg·kg–1), with β-sitosterol being the major sterol in all the samples with percentages between 65 and 83%.El objetivo de este estudio fue la caracterización de los componentes menores presentes en los aceites obtenidos del mesocarpio de frutos de especies de bacaba (Oenocarpus bacaba), buriti (Mauritia flexuosa), inajá (Maximiliana maripa), pupuña (Bactris gasipaes) y tucumá (Astrocaryum vulgare), de importante producción en el Estado de Amapá, Brasil. Se determinaron las dos principales fracciones presentes en los aceites. Por una parte, los compuestos menores derivados de los componentes mayoritarios o triglicéridos (TAG): dímeros de TAG, TAG oxidados y diglicéridos (DAG) relacionados con la calidad de los aceites y, por otra, los principales grupos presentes en la fracción insaponificable (hidrocarburos, alcoholes alifáticos, esteroles y tocoferoles) relacionados con la calidad de los aceites. Los resultados indicaron que todos los aceites extraídos tenían buena calidad inicial, siendo los DAG los mayoritarios entre los compuestos menores glicerídicos. La concentración de hidrocarburos (50-734 mg·kg–1) y de alcoholes alifáticos (80-490 mg·kg–1) fue muy variable correspondiendo al aceite de inajá el mayor contenido en hidrocarburos y en alcoholes. En el caso de los tocoferoles, las mayores cantidades correspondieron a los aceites de buriti (1567 mg·kg–1) y tucumá (483 mg·kg–1) y la presencia de cantidades significativas de tocotrienoles sólo se detectaron en aceite de inajá. Finalmente, se encontraron concentraciones elevadas de esteroles en todas las muestras, especialmente en los aceites de pupuña (4456 mg·kg–1) y tucumá (2708 mg·kg–1), siendo el β-sitosterol el esterol mayoritario con porcentajes entre 65 y 83%

Mixed methods approach to understanding farmer and agricultural advisor perceptions of climate change and adaptation in Vermont, United States

The relationships among farmers’ belief in climate change, perceptions of climate-related risk, and use of climate adaptation practices is a growing topic of interest in U.S. scholarship. The northeast region is not well represented in the literature, although it is highly agricultural and will likely face climate-related risks that differ from those faced in other regions. We used a mixed methods approach to examine northeast farmers’ perceptions of climate change and climate-related risks over time, and perceived trade-offs associated with on-farm practices. Our investigation shows how northeastern farmers think about climate-risk, and what they are doing to address it

SDRS: a new lossless dimensionality reduction for text corpora

In recent years, most content-based spam filters have been implemented using Machine Learning (ML) approaches by means of token-based representations of textual contents. After introducing multiple performance enhancements, the impact has been virtually irrelevant. Recent studies have introduced synset-based content representations as a reliable way to improve classification, as well as different forms to take advantage of semantic information to address problems, such as dimensionality reduction. These preliminary solutions present some limitations and enforce simplifications that must be gradually redefined in order to obtain significant improvements in spam content filtering. This study addresses the problem of feature reduction by introducing a new semantic-based proposal (SDRS) that avoids losing knowledge (lossless). Synset-features can be semantically grouped by taking advantage of taxonomic relations (mainly hypernyms) provided by BabelNet ontological dictionary (e.g. “Viagra” and “Cialis” can be summarized into the single features “anti-impotence drug”, “drug” or “chemical substance” depending on the generalization of 1, 2 or 3 levels). In order to decide how many levels should be used to generalize each synset of a dataset, our proposal takes advantage of Multi-Objective Evolutionary Algorithms (MOEA) and particularly, of the Non-dominated Sorting Genetic Algorithm (NSGA-II). We have compared the performance achieved by a Naïve Bayes classifier, using both token-based and synset-based dataset representations, with and without executing dimensional reductions. As a result, our lossless semantic reduction strategy was able to find optimal semantic-based feature grouping strategies for the input texts, leading to a better performance of Naïve Bayes classifiers.info:eu-repo/semantics/acceptedVersio

Effect of initial conditions on the speed of reaction-diffusion fronts

The effect of initial conditions on the speed of propagating fronts in reaction-diffusion equations is examined in the framework of the Hamilton-Jacobi theory. We study the transition between quenched and nonquenched fronts both analytically and numerically for parabolic and hyperbolic reaction diffusion. Nonhomogeneous media are also analyzed and the effect of algebraic initial conditions is also discussed

Relic gravitational waves and present accelerated expansion

We calculate the current power spectrum of the gravitational waves created at the big bang (and later amplified by the different transitions during the Universe expansion) taking into account the present stage of accelerated expansion. Likewise, we determine the power spectrum in a hypothetical second dust era that would follow the present one if at some future time the dark energy, that supposedly drives the current accelerated expansion, evolved in such a way that it became dynamically equivalent to cold dark matter. The calculated power spectrum as well as the evolution of the density parameter of the waves may serve to discriminate between phases of expansion and may help ascertain the nature of dark energy.Comment: 20 pages, uses revtex4, 1 figure ps and 3 figures eps. To be published in Physical Review

XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence

We report on XMM-Newton observations performed on 2001 September 13-14 of the neutron star X-ray transient KS 1731-260 in quiescence. The source was detected at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10 keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001 quiescent flux of KS 1731-260 is lower than that observed during the Chandra observation in March 2001. In the cooling neutron star model for the quiescent X-ray emission of neutron star X-ray transients, this decrease in the quiescent flux implies that the crust of the neutron star in KS 1731-260 cooled down rapidly between the two epochs, indicating that the crust has a high conductivity. Furthermore, enhanced cooling in the neutron star core is also favored by our results.Comment: Accepter for publication in ApJ Letters, 22 May 200