Leisure and the Opportunity Cost of Travel Time in Recreation Demand Analysis: A Re-Examination

Using count data models that account for zero-truncation, overdispersion, and endogenous stratification, this paper estimates the value of access to recreational parks. The focus is on the valuation of the opportunity cost of travel time within the cost of the trip and its effects on estimated consumer surplus. The fraction of hourly earnings that corresponds to the opportunity cost of travel time is endogenously estimated as a function of visitor characteristics, rather than fixed exogenously. We find that the relevant opportunity cost of time for most visitors represents a smaller fraction of their wage rate than commonly assumed previously.value of time; endogenous stratification; on-site sampling; overdispersion; recreation demand; travel cost method

Strong asymmetry for surface modes in nonlinear lattices with long-range coupling

We analyze the formation of localized surface modes on a nonlinear cubic waveguide array in the presence of exponentially-decreasing long-range interactions. We find that the long-range coupling induces a strong asymmetry between the focusing and defocusing cases for the topology of the surface modes and also for the minimum power needed to generate them. In particular, for the defocusing case, there is an upper power threshold for exciting staggered modes, which depends strongly on the long-range coupling strength. The power threshold for dynamical excitation of surface modes increase (decrease) with the strength of long-range coupling for the focusing (defocusing) cases. These effects seem to be generic for discrete lattices with long-range interactions.Comment: 4 pages, 5 figures, submitted for publicatio

Graphene Multi-Protonation: a Cooperative Mechanism for Proton Permeation

The interaction between protons and graphene is attracting a large interest due to recent experiments showing that these charged species permeate through the 2D material following a low barrier (~ 0.8 eV) activated process. A possible explanation involves the flipping of a chemisorbed proton (rotation of the C-H$^+$ bond from one to the other side of the carbon layer) and previous studies have found so far that the energy barriers (around 3.5 eV) are too high to explain the experimental findings. Contrarily to the previously adopted model assuming an isolated proton, in this work we consider protonated graphene at high local coverage and explore the role played by nearby chemisorbed protons in the permeation process. By means of density functional theory calculations exploiting large molecular prototypes for graphene it is found that, when various protons are adsorbed on the same carbon hexagonal ring, the permeation barrier can be reduced down to 1.0 eV. The related mechanism is described in detail and could shed a new light on the interpretation of the experimental observations for proton permeation through graphene.Comment: 16 pages, 5 figure

Three-body hadronic structure of low-lying 1/2+1/2^+ Σ\Sigma and Λ\Lambda resonances

We discuss the dynamical generation of some low-lying $1/2^+$ $\Sigma$'s and $\Lambda$'s in two-meson one-baryon systems. These systems have been constructed by adding a pion in $S$-wave to the $\bar{K} N$ pair and its coupled channels, where the $1/2^-$ $\Lambda$(1405)-resonance gets dynamically generated. We solve Faddeev equations in the coupled-channel approach to calculate the $T$-matrix for these systems as a function of the total energy and the invariant mass of one of the meson-baryon pairs. This squared $T$-matrix shows peaks at the energies very close to the masses of the strangeness -1, $1/2^+$ resonances listed in the particle data book.Comment: Contribution to the NSTAR07 conferenc

Long-Term Temporal Analysis of the Human Fecal Microbiota Revealed a Stable Core of Dominant Bacterial Species

Next-generation sequencing has greatly contributed to an improved ecological understanding of the human gut microbiota. Nevertheless, questions remain regarding the characteristics of this ecosystem and the ecological processes that shape it, and controversy has arisen regarding the stability of the bacterial populations and the existence of a temporal core. In this study, we have characterized the fecal microbial communities of three human individuals over a one-year period by 454 pyrosequencing of 16S rRNA tags in order to investigate the temporal characteristics of the bacterial communities. The findings revealed a temporal core of 33 to 40 species-level Operational Taxonomic Units (OTUs) within subjects. Although these OTUs accounted only for around 12% of the total OTUs detected, they added up to \u3e75% of the total sequences obtained for each individual. In order to determine the capacity of the sequencing and bioinformatic approaches applied during this study to accurately determine the proportion of a core microbiota, we analyzed the fecal microbiota of nine mice with a defined three-member community. This experiment revealed that the sequencing approach inflated the amount of rare OTUs, which introduced a significant degree of artificial variation across samples, and hence reduced the apparent fraction of shared OTUs. However, when assessing the data quantitatively by focusing on dominant lineages, the sequencing approaches deliver an accurate representation of the community. In conclusion, this study revealed that the human fecal microbiota is dominated by around 40 species that maintain persistent populations over the duration of one year. The findings allow conclusions about the ecological factors that shape the community and support the concept of a homeostatic ecosystem controlled largely by deterministic processes. Our analysis of a three-member community revealed that methodological artifacts of OTU-based approaches complicate core calculations, and these limitations have to be considered in the interpretation of microbiome studies

Band structure of topological insulators from noise measurements in tunnel junctions

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al$_2$O$_3$/Co tunnel junctions with bottom TI electrodes of either Bi$_2$Te$_3$ or Bi$_2$Se$_3$. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems, that can further lead to the manipulation of their spin-polarized properties for technological purposes

LPV methods for fault-tolerant vehicle dynamic control

International audienceThis paper aims at presenting the interest of the Linear Parameter Varying methods for vehicle dynamics control, in particular when some actuators may be in failure. The cases of the semi-active suspension control problem and the yaw control using braking, steering and suspension actuators will be presented. In the first part, we will consider the semi-active suspension control problem, where some sensors or actuator (damper leakage) faults are considered. From a quarter-car vehicle model including a non linear semi-active damper model, an LPV model will be described, accounting for some actuator fault represented as some varying parameters. A single LPV fault-tolerant control approach is then developed to manage the system performances and constraints. In the second part the synthesis of a robust gain-scheduled H1 MIMO vehicle dynamic stability controller (VDSC), involving front steering, rear braking, and four active suspension actuators, is proposed to improve the yaw stability and lateral performances. An original LPV method for actuator coordination is proposed, when the actuator limitations and eventually failures, are taken into account. Some simulations on a complex full vehicle model (which has been validated on a real car), subject to critical driving situations (in particular a loss of some actuator), show the efficiency and robustness of the proposed solution

Essays on the political economy of development

The present collection of essays studies some of the ways in which the interaction of economic and political forces affects a country’s development path. The focus of the thesis is on Colombia, which is a fertile setting for the study of the political economy of development given its long-lasting internal conflict, the multiple reforms to the functioning of the state that have taken place in the last decades and the availability of high quality sub-national data. The first two chapters explore people’s tendency to use resources differently depending on their source. The first chapter shows how the source of public revenue affects a government’s incentives to provide public goods and services, while the second one studies people’s propensity to make risky choices when playing a game with easily-gotten house money. The third chapter contributes to our understanding of the international dimension of civil conflict by analyzing the effects of access to territory in a neighboring country on the intensity of an insurgent group’s activities. The idea that governments perform better when they are funded with tax revenue has a long history and surfaces often in debates regarding the origin of the natural resource curse, the effectiveness of foreign aid and the benefits from decentralization. However, the empirical evidence backing this claim is somewhat limited. In the first chapter, I try to fill this gap by comparing the effects of increases in internally-raised tax revenue and in royalties from the extraction of oil on local public good provision in a panel of Colombian municipalities. I find that tax revenue leads to an improvement in public services while oil royalties have no effect. Furthermore, I document a negative effect of royalties on the quality of government, as measured by the disciplinary prosecution of local public officials. One possible explanation for the results in chapter 1 is that taxation leads to greater accountability because voters value tax revenue more than revenue from an external source. The idea that people assign greater value to resources over which they have some sense of ownership is further explored in the second chapter. In that chapter, which is the result of joint work with Juan Camilo C´ardenas, Nicol´as De Roux and Christian Jaramillo, we show that the risk aversion displayed by participants in a lab experiment varies depending on whether they received the endowment on the same day of the session or one month in advance. We interpret this finding as evidence of people’s reduced risk aversion when allocating easily-gotten resources, also known as the ‘housemoney’ effect. In the third chapter, I turn my attention to Colombia’s internal armed conflict and I study the allegation that the administration of Hugo Ch´avez provided access to territory in neighboring Venezuela to Colombian insurgent groups FARC and ELN. I document a disproportionate increase in the intensity of insurgent activity (mainly by FARC) in Colombian municipalities next to the border with Venezuela after Ch´avez comes to power in 1999. This finding is consistent with the idea that the rebels had access to a safe haven across the border during the Ch´avez administration, but that the strategic advantage provided by this sanctuary decreased with distance to the border. This chapter contributes to our understanding of foreign support for insurgent groups by developing a novel data-driven method for the detection of the usually secretive activities of trans-national rebel groups. It additionally provides credible estimates of the causal effect of access to foreign territory on insurgent activities