Realistic Version Research Articles

Population, Population Density, and Technological Change

In a model on population and endogenous technological change, Kremer combines a short-run Malthusian scenario where income determines the population that can be sustained, with the Boserupian insight that greater population spurs technological change and can therefore lift a country out of its Malthusian trap. We show that a more realistic version of the model, which combines population and population density, allows deeper insights into these processes. The incorporation of population density also allows a superior interpretation of the empirical regularities between the level of population, population density, population growth, and economic development, both at aggregated and disaggregated levels.

Sensitivity of Model-Generated Daytime Surface Heat Fluxes over Snow to Land-Cover Changes

Snow cover can significantly suppress daytime temperatures by increasing the surface albedo and limiting the surface temperature to 0°C. The strength of this effect is dependent upon how well the snow can cover, or mask, the underlying surface. In regions where tall vegetation protrudes through a shallow layer of snow, the temperature-reducing effects of the snow will be suppressed since the protruding vegetation will absorb solar radiation and emit an upward turbulent heat flux. This means that an atmospheric model must have a reasonable representation of the land cover, as well as be able to correctly calculate snow depth, if an accurate simulation of surface heat fluxes, air temperatures, and boundary layer structure is to be made. If too much vegetation protrudes through the snow, then the surface sensible heat flux will be too large and the air temperatures will be too high. In this study four simulations are run with the Regional Atmospheric Modeling System (RAMS 4.30) for a snow event that occurred in 1988 over the Texas Panhandle. The first simulation, called the control, is run with the most realistic version of the current land cover and the results verified against both ground stations and aircraft data. Simulations 2 and 3 use the default methods of specifying land cover in RAMS 4.29 and RAMS 4.30, respectively. The significance of these variations in land-cover definition is then examined by comparing with the control run. Finally, the last simulation is run with the land cover defined as all short grass, the natural cover for the region. The results of this study indicate that variations in the land-cover specification can lead to differences in sensible heat flux over snow as large as 80 W m−2. These differences in sensible heat flux can then lead to differences in daytime temperatures of as much as 6°C. Also, the height of the afternoon boundary layer can vary by as much as 200–300 m. In addition, the results suggest that daytime temperatures are cooler over snow in the regions where short grass has been converted to cropland, while they appear to be warmer over regions where shrubs have increased.

Vulnerability studies of high-speed compulsators

The utilization of high-speed compulsators is central to the success of the electromagnetic pulsed power launch systems under development. The efficient use of these high-energy storage devices depends, in turn, on high-strength carbon composite bands for attaining sufficient speed, and thus energy levels, without disintegrating. While advances in material science allowed significant improvements in the performance of the latest generation of compulsators, a growing concern is their vulnerability to ballistic impact. A comprehensive experimental study using a scaled but realistic version of an operational compulsator is presented. A set of two test rotors consisting of several layers, including a simulated coil section and an outer carbon confining shell, was tested. The rotors were spun to approximately 18 000 r/min in a high-speed test facility, and shaped charges were fired at the outer shell at normal incidence. The results of a set of experiments are presented, showing damage ranging from benign to complete failure, depending on the impact energy level.

Introduction: Reprioritizing the role of science in a realistic version of the scientist-practitioner model.

This series of articles attempts to redefine and reprioritize the role of science in search for a clinically meaningful and realistic version of the scientist-practitioner (S-P) model. To this end, science is discussed as both research-driven practice and clinically meaningful research, keeping practitioners and their needs in the center of attention. In this process, common problems in the actualization of the S-P model are discussed, and potential solutions are offered.

Equilibrium and nonequilibrium solitons in a lossy split-step system with lumped amplification

We propose a more realistic version of the recently introduced split-step model (SSM), which consists of periodically alternating dispersive and nonlinear segments, by adding uniformly distributed loss and lumped gain to it. In the case when the loss is exactly balanced by gain, a family of stable equilibrium solitons (ESs) is found. Unless the system's period L is very small, saturation is observed in the dependence of the amplitude of the established ES vs. that of the initial pulse. Stable nonequilibrium solitons (NESs) are found in the case when the gain slightly exceeds (by up to ≃3%) the value necessary to balance the loss. The existence of NESs is possible as the excessive energy pump is offset by permanent radiation losses, which is confirmed by computation of the corresponding Poynting vector. Unlike ESs that form a continuous family of solutions, NES is an isolated solution, which disappears in the limit L→0, i.e., it cannot be found in the overpumped nonlinear Schrödinger equation. Interactions between ESs turn out to be essentially the same as in SSM without loss and gain, while interactions between NESs are different: two NESs perturb each other by the radiation jets emanating from them, even if they are separated by a large distance. Moving NESs survive collisions, changing their velocities.

Spatially Structured Metapopulation Models: Global and Local Assessment of Metapopulation Capacity

We model metapopulation dynamics in finite networks of discrete habitat patches with given areas and spatial locations. We define and analyze two simple and ecologically intuitive measures of the capacity of the habitat patch network to support a viable metapopulation. Metapopulation persistence capacity λM defines the threshold condition for long-term metapopulation persistence as λM>δ, where δ is defined by the extinction and colonization rate parameters of the focal species. Metapopulation invasion capacity λI sets the condition for successful invasion of an empty network from one small local population as λI>δ. The metapopulation capacities λM and λI are defined as the leading eigenvalue or a comparable quantity of an appropriate “landscape” matrix. Based on these definitions, we present a classification of a very general class of deterministic, continuous-time and discrete-time metapopulation models. Two specific models are analyzed in greater detail: a spatially realistic version of the continuous-time Levins model and the discrete-time incidence function model with propagule size-dependent colonization rate and a rescue effect. In both models we assume that the extinction rate increases with decreasing patch area and that the colonization rate increases with patch connectivity. In the spatially realistic Levins model, the two types of metapopulation capacities coincide, whereas the incidence function model possesses a strong Allee effect characterized by λI=0. For these two models, we show that the metapopulation capacities can be considered as simple sums of contributions from individual habitat patches, given by the elements of the leading eigenvector or comparable quantities. We may therefore assess the significance of particular habitat patches, including new patches that might be added to the network, for the metapopulation capacities of the network as a whole. We derive useful approximations for both the threshold conditions and the equilibrium states in the two models. The metapopulation capacities and the measures of the dynamic significance of particular patches can be calculated for real patch networks for applications in metapopulation ecology, landscape ecology, and conservation biology.

Radio Tomography (RTFEM), Practical Image Fidelity or Resolution

Radio Tomography (RTFEM) is applied in mine to define geological hazards in advance of coal mining, outline ore bodies in base metal exploration and a range of geotechnical and environmental applications. This paper ranks the dominant sources of image degradation in terms of significance, so as to provide guidelines on the practical applicability of the technique. RTFEM appears to have niche rather than general applicability: it cannot accurately image features oriented parallel to the survey line; it will not generally be able to probe the interior of targets unless the attenuation is such that signal is observable through the strongly attenuated zone (ie above noise level); The resolution perpendicular to the access lines will always be relatively poor. A realistic version of the actual conductivity (or attenuation) image can be achieved by using robust imaging inversion methods. For base metal imaging, it would also be desirable to develop algorithms which can deal better with signal attenuated below the noise level at the receiver, and which can take better account of the variation in antenna coupling. The current approach?s still have significant limitations. The technique is applicable for coal seam imaging, but resolution parallel to data collection lines is two to three times less. In these cases usually the only feature observable is a step in image level and not the outline of a structure. The technique is well suited for delineating the external geometry of base metal deposits when the host rock is relatively homogeneous and resistive. This precludes working in areas where the host rock is pervasively altered or pervasively mineralised with disseminated sulphides.

Some problems in the generalized theory of finite Fermi systems

A realistic version of the generalization of the theory of finite Fermi systems to the case where some complex configurations involving phonons are explicitly taken into account is proposed. Secular equations describing the fragmentation of simple states in odd and even-even nuclei over complex configurations that belong to, respectively, the quasiparticle ⊗ phonon + quasiparticle ⊗ phonon ⊗ phonon and the two quasiparticles ⊗ phonon type and which are presently of greatest interest are derived on the basis of general relations for nuclei that involve pairing (nonmagic nuclei). These equations take into account effects associated with ground-state correlations due to complex configurations and with the additional quasiparticle-phonon mechanism of Cooper pairing in nuclei. The effects in question were disregarded previously, but they are of interest since they can be observed in present-day experiments.

Hebbian learning and temporary storage in the convergence-zone model of episodic memory

The convergence-zone model shows how sparse, random memory patterns can lead to one-shot storage and high capacity in the hippocampal component of the episodic memory system. This paper presents a biologically more realistic version of the model, with continuously weighted connections and storage through Hebbian learning and normalization. In contrast to the gradual weight adaptation in many neural network models, episodic memory turns out to require high learning rates. Normalization allows earlier patterns to be overwritten, introducing time-dependent forgetting similar to the hippocampus.

Sonoluminescence as a QED vacuum effect. II. Finite volume effects

In a companion paper [quant-ph/9904013] we have investigated several variations of Schwinger's proposed mechanism for sonoluminescence. We demonstrated that any realistic version of Schwinger's mechanism must depend on extremely rapid (femtosecond) changes in refractive index, and discussed ways in which this might be physically plausible. To keep that discussion tractable, the technical computations in that paper were limited to the case of a homogeneous dielectric medium. In this paper we investigate the additional complications introduced by finite-volume effects. The basic physical scenario remains the same, but we now deal with finite spherical bubbles, and so must decompose the electromagnetic field into Spherical Harmonics and Bessel functions. We demonstrate how to set up the formalism for calculating Bogolubov coefficients in the sudden approximation, and show that we qualitatively retain the results previously obtained using the homogeneous-dielectric (infinite volume) approximation.

Anti-Arrhenius behaviour in positronium chemistry: a Kramers' turnover?

The observed anti-Arrhenius temperature dependence of the rate for positronium–acceptor (Ps–Ac) complex formation above, and a normal Arrhenius behaviour below, a solvent-dependent critical temperature T 0, is argued to be ascribable to a Kramers' turnover, relating to viscosity effects unleashed by temperature-dependent surface tension, in the context of the bubble model of the positronium atom in liquids, provided one uses a more realistic and robust version thereof developed by us, which is also in closer accord with lifetime and gamma–gamma angular correlation data in pure solvents.

On the Distributional Effects of Social Security Reform

How will the distribution of welfare, consumption, and leisure across households be affected by social security reform? This paper addresses this question for social security reforms with a two-tier structure by comparing steady states under a realistic version of the current U.S. system and under the two-tier system. The first tier is a mandatory, defined-contribution pension offering a retirement annuity proportional to the value of taxes paid, whereas the second tier guarantees a minimum retirement income. Our findings, which are summarized in the Introduction, do not in general favor the implementation of pay-as-you go versions of the two-tier system for the U.S. economy. Journal of Economic Literature Classification Numbers: D3, E6.

Asymmetric Dynamics of Rent and Vacancy Rates in the Tokyo Office Market

In recent years, it has been shown that the dynamics of office markets are asymmetric depending on the market conditions and the direction of supply and demand shocks. However, the actual state of asymmetry varies significantly from market to market, and an overview of the discussion is needed. In this study, we test our hypothesis on asymmetric dynamics in the Tokyo office market, one of the world's largest markets. We employ the rent-adjustment process model proposed by Englund et al. (2008), an improved and more realistic version of the error correction model that captures the interaction between rent, vacancy rates, and stock. The data of the Tokyo office market range from January 2000 to September 2015 and cover ten regions. The results reveal that the mechanism of rent and vacancy rate fluctuation depends largely on the direction of change in supply and demand and on market conditions, especially the upward and downward movements of rents. It is also shown that increases in demand and supply not only encourage rents toward equilibrium, but also have the effect of overshooting them. These results can be valuable in properly capturing future shocks in demand and supply.

Probability and Choice in the Selection Task

Two experiments using a realistic version of the selection task examined the relationship between participants' probability estimates of finding a counter example and their selections. Experiment 1 used everyday categories in the context of a scenario to determine whether or not the number of instances in a category affected the estimated probability of a counter-example. Experiment 2 modified the scenario in order to alter participants' estimates of finding a specific counter-example. Unlike Kirby 1994a, but consistent with his proposals, both studies showed that probability estimates significantly predicted selection. Overall results point to the value of understanding selections in terms of their subjective expected utility.

Earthquake statistics and fractal faults

We introduce a Self-affine Asperity Model (SAM) for the seismicity that mimics the fault friction by means of two fractional Brownian profiles (fBm) that slide one over the other. An earthquake occurs when there is an overlap of the two profiles representing the two fault faces and its energy is assumed proportional to the overlap surface. The SAM exhibits the Gutenberg-Richter law with an exponent $\beta$ related to the roughness index of the profiles. Apart from being analytically treatable, the model exhibits a non-trivial clustering in the spatio-temporal distribution of epicenters that strongly resembles the experimentally observed one. A generalized and more realistic version of the model exhibits the Omori scaling for the distribution of the aftershocks. The SAM lies in a different perspective with respect to usual models for seismicity. In this case, in fact, the critical behaviour is not Self-Organized but stems from the fractal geometry of the faults, which, on its turn, is supposed to arise as a consequence of geological processes on very long time scales with respect to the seismic dynamics. The explicit introduction of the fault geometry, as an active element of this complex phenomenology, represents the real novelty of our approach.

Scheduling projects to maximize net present value — the case of time-dependent, contingent cash flows

The problem of scheduling activities in a project to maximize its Net Present Value (NPV) has been solved for the case where net cash flow magnitudes are independent of the time of realization. This paper models a more realistic version of this problem — because of incentive payments and penalties for early and late event occurrences, respectively, and because of changing costs of resources over time, net cash flow magnitudes are dependent on the time of realization. We formulate an optimization program for this more general problem and present a simulated annealing solution approach. We test different implementation strategies for this algorithm and suggest a method for choosing neighborhood moves. We compare the NPVs of the solutions obtained from our formulation with the NPVs of early start schedules and with late start schedules for 168 different problems. These computational results show that the simulated annealing approach consistently produces substantially better solutions than the early start or late start schedules. Even poor simulated annealing neighborhood moves give improved solutions for most problems studied.

Scientific Realism and the ‘Pessimistic Induction’

Over the last two decades, the debate over scientific realism has been dominated by two arguments that pull in contrary directions: the ‘no miracle’ argument and the ‘pessimistic induction’. The latter suggests that the historical record destroys the realist’s belief in an explanatory connection between truthlikeness and genuine empirical success. This paper analyzes the structure of the ‘pessimistic induction’, presents a move—the divide et impera move—that neutralizes it, and motivates a substantive yet realistic version of scientific realism. This move is also compared with Worrall’s and Kitcher’s recent reactions to the ‘pessimistic induction’.

A Diagnostic Barotropic Finite-Element Ocean Circulation Model

Abstract The finite-element method possesses many advantages over more traditional numerical techniques used to solve systems of differential equations. These advantages include a number of conservation properties and a natural treatment of boundary conditions. The method's piecewise nature makes it useful when dealing with irregular domains and similarly when using variable horizontal resolution. To take advantage of these properties, a finite-element representation of the linearized, steady-state, barotropic potential vorticity equation is developed. The Stommel problem is used as an initial test for the model. A fourth-order eddy viscosity term is then added, and the resulting problem is solved in both simply and multiply connected domains under both slip and no-slip boundary conditions. The beta-plane assumption is then relaxed, and the model is reformulated in spherical coordinates. A realistic geography and topography version of this model is also used to examine the barotropic circulation in the No...

A fuzzy surface cartographic representation for forestry based on Voronoi diagram area stealing

A fuzzy surface map representation for a forested area in Quebec was developed and evaluated relative to ground truth for two forest parameters (basal area and species composition). The fuzzy surface representation was found to perform slightly better than a conventional thematic map relative to both parameters. The fuzzy surfaces were also better at estimating species composition than basal area. It is suggested that because the fuzzy surface representation has the potential to show local variations, and to show boundaries between forest types as transition zones, it is a more realistic and useful representation of "the real world" in forestry than are conventional thematic maps. With refinement, the fuzzy surface representation developed has the potential to decrease the amount of simplification required in the map construction process and store a more realistic version of a forest in a geographic information system, thereby allowing practitioners to be closer to analyzing real-world conditions than is presently possible.

A realistic version of the Y model for DNA dynamics and selection of soliton speed

In the Y model of DNA dynamics [Phys. Lett. A 136 (1989) 413] one assumes the bases to be all equal; in this note we modify the Y model to account for the differences among bases. We use this “realistic Y model” to study significant special sequences, and obtain a selection of the speed of solitons - except the simplest, “elementary” ones - in DNA; the selected speed is unphysical, suggesting that only the elementary solitons are present in real DNA. The obtained results also apply to a purine/pyrimidine approximation for the general case. We also discuss how cooperative effects could cause the equal bases approximation to the better than the purine/pyrimidine one.