Periodic and discrete Zak bases

Weyl's displacement operators for position and momentum commute if the product of the elementary displacements equals Planck's constant. Then, their common eigenstates constitute the Zak basis, each state specified by two phase parameters. Upon enforcing a periodic dependence on the phases, one gets a one-to-one mapping of the Hilbert space on the line onto the Hilbert space on the torus. The Fourier coefficients of the periodic Zak bases make up the discrete Zak bases. The two bases are mutually unbiased. We study these bases in detail, including a brief discussion of their relation to Aharonov's modular operators, and mention how they can be used to associate with the single degree of freedom of the line a pair of genuine qubits.Comment: 15 pages, 3 figures; displayed abstract is shortened, see the paper for the complete abstrac

Exact Random Walk Distributions using Noncommutative Geometry

Using the results obtained by the non commutative geometry techniques applied to the Harper equation, we derive the areas distribution of random walks of length $N$ on a two-dimensional square lattice for large $N$, taking into account finite size contributions.Comment: Latex, 3 pages, 1 figure, to be published in J. Phys. A : Math. Ge

Regeneration Strategies and Forest Resilience to Changing Fire Regimes: Insights From a Goldilocks Model

Disturbances are ubiquitous in ecological systems, and species have evolved a range of strategies to resist or rebound following disturbance. Understanding how the presence and complementarity of regeneration traits will affect community responses to disturbance is increasingly urgent as disturbance regimes shift beyond their historical ranges of variability. We define disturbance niche as a species\u27 fitness across a range of disturbance sizes and frequencies that can reflect the fundamental or realized niche, that is, whether the species occurs alone or with other species. We developed a model of intermediate complexity (i.e., a Goldilocks model) to infer the disturbance niche. We parameterized the model for subalpine forests in Yellowstone National Park (USA) adapted to infrequent stand-replacing fires and included the three major tree-regeneration strategies: (1) obligate seeders that rely on ex situ seeding into burned areas (non-serotinous lodgepole pine, Pinus contorta var. latifola), (2) obligate seeders that depend on in situ seedbanks (serotinous lodgepole pine, Pinus contorta var. latifola), and (3) species that can resprout from surviving roots following fire (quaking aspen, Populus tremuloides). Our results showed which regeneration strategies increase or decrease in prevalence as fire rotation declines. Non-serotinous pines were extirpated when fire rotation was below 50 years in a monoculture and 100 years in a mixed forest; serotinous pines were extirpated when fire rotation was below 20 years; and aspen was extirpated when fire rotation fell below 6 years. The fundamental and realized disturbance niches pinpointed the key mechanisms limiting regeneration for each strategy, namely, increasing fire size for non-serotinous pine (ex situ seeders), decreasing fire frequency for serotinous pine (in situ seeders), and interspecific competition for aspen (resprouters). In a mixed forest, the three regeneration strategies were complementary and each dominated at different combinations of fire size and frequency. Consequently, diversity of regeneration strategies enhanced forest resilience to declining fire rotations. Despite its simplicity, our Goldilocks model produced realistic dynamics and could be readily adapted to other disturbance-prone ecosystems to explore the generality of these results. The disturbance niche is a key concept for anticipating community resilience to changing disturbance regimes

Biochemical correlates of cardiac hypertrophy. III. Changes in DNA content. The relative contributions of polyploidy and mitotic activity

DNA concentration in the left ventricle of 3-month-old rats 3 to 12 days after aortic constriction was similar to the values obtained in sham-operated litter mates (1.38±0.03 µg/mg wet weight). The total DNA content of the heart was thus increased proportionally to its enlargement. In chronic hypertrophy (2 to 4 months after aortic constriction), the DNA content was increased to a significantly less extent than in short-term hypertrophy. The percentage of polyploid nuclei in 12- to 13-week-old rats increased 1 to 3 weeks after aortic constriction from a normal value of 1.8% to 4.2%. In 17- to 18-week-old rats, polyploidy was present in 3.9% of the nuclei 10 days after banding as compared with 1.6% in normal rats. Mitotic figures were localized almost exclusively outside muscle cells. Mitotic indices in hypertrophied hearts were ten times higher than in controls during the first 3 weeks after aortic constriction

A Layered-Manufacturing Process For the Fabrication of Glass-Fiber-Reinforced Composites

In this paper, we present a rapid manufacturing process for the layered fabrication of polymer-based composite parts using short discontinuous fibers as reinforcements. In the recent past, numerous research efforts, similar to ours, have been made to produce fiber-reinforced plastic parts via layered manufacturing methods. However, most of these attempts have not resulted in the development of an effective commercially-viable manufacturing process. Our proposed fabrication process on the other hand has been experimentally verified to yield composite parts comparable in quality to pure polymer parts manufactured on a commercial stereolithography system.Mechanical Engineerin

Wave-packet dynamics in slowly perturbed crystals: Gradient corrections and Berry-phase effects

We present a unified theory for wave-packet dynamics of electrons in crystals subject to perturbations varying slowly in space and time. We derive the wave-packet energy up to the first order gradient correction and obtain all kinds of Berry-phase terms for the semiclassical dynamics and the quantization rule. For electromagnetic perturbations, we recover the orbital magnetization energy and the anomalous velocity purely within a single-band picture without invoking inter-band couplings. For deformations in crystals, besides a deformation potential, we obtain a Berry-phase term in the Lagrangian due to lattice tracking, which gives rise to new terms in the expressions for the wave-packet velocity and the semiclassical force. For multiple-valued displacement fields surrounding dislocations, this term manifests as a Berry phase, which we show to be proportional to the Burgers vector around each dislocation.Comment: 12 pages, RevTe

Do subtropical seasonal forests in the Gran Chaco, Argentina, have a future?

While much information is available about tropical and temperate ecosystems, there is a remarkably little information as to land cover and land use changes in the subtropical biomes of the world. Here, we quantify changes in the spatial patterns of land cover types at the southern edge of the seasonally dry, subtropical Chaco forest of South America during the second half of the 20th century using a vegetation map printed in 1969 and a Landsat TM based digital map produced 30 years later. Results show a massive contraction of forest; ca. 1.2 million ha of original lowland and mountain subtropical dry forests and woodlands, 85% of the total, have been cleared in only 30 years. This loss of Chaco forests of 2.2% year1 is consistent with or even exceeds, global trends. Forest vegetation now persists as fragments where there was formerly continuous cover. Most of undisturbed Chaco forest has now been converted to pasture or is undergoing secondary succession. Today, these new vegetation types, resulting mainly from agricultural expansion, have increased 10-fold in cover and now represent the commonest land cover types. The increased intensity of agricultural usage, possibly triggered by an increase in annual rainfall during the last decades, has been accompanied by changes in agricultural practices and a relative decline in the rural population.Fil: Zak, Marcelo Román. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Cabido, Marcelo Ruben. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Hodgson, John G.. Peak Science and Environment, Station House; Reino Unido. University of Sheffield; Reino Unid

Documentation of Atmospheric Conditions During Observed Rising Aircraft Wakes

Flight tests were conducted in the fall of 1995 off the coast of Wallops Island, Virginia in order to determine characteristics of wake vortices at flight altitudes. A NASA Wallops Flight Facility C130 aircraft equipped with smoke generators produced visible wakes at altitudes ranging from 775 to 2225 m in a variety of atmospheric conditions, orientations (head wind, cross wind), and airspeeds. Meteorological and aircraft parameters were collected continuously from a Langley Research Center OV-10A aircraft as it flew alongside and through the wake vortices at varying distances behind the C130. Meteorological data were also obtained from special balloon observations made at Wallops. Differential GPS capabilities were on each aircraft from which accurate altitude profiles were obtained. Vortices were observed to rise at distances beyond a mile behind the C130. The maximum altitude was 150 m above the C130 in a near neutral atmosphere with significant turbulence. This occurred from large vertical oscillations in the wakes. There were several cases when vortices did not descend after a very short initial period and remained near generation altitude in a variety of moderately stable atmospheres and wind shears

Observed Changes in Atmospheric Boundary Layer Properties at Memphis International Airport During August 1995

As part of the NASA Terminal Area Productivity Program, Langley Research Center embarked on a series of field measurements of wake vortex characteristics and associated atmospheric boundary layer properties. One measurement period was at the Memphis International Airport in August 1995. Atmospheric temperature, humidity, winds, turbulence, radiation, and soil properties were measured from a variety of sensor systems and platforms including sodars, profilers, aircraft and towers. This research focused on: (1) changes that occurred in tower data during sunrise and sunset transitions, (2) vertical variation of temperature and cross-head winds at selected times utilizing combinations of sensors, and (3) changes measured by an OV-10 aircraft during approaches and level flights. Significant but not unusual changes are documented and discussed in terms of expected boundary layer behavior. Questions on measurement and prediction of these changes from existing and near-term capabilities are discussed in the context of a future Aircraft Vortex Spacing System

Biochemical correlates of cardiac hypertrophy. IV. Observations on the cellular organization of growth during myocardial hypertrophy in the rat

The mechanisms by which the DNA content of the heart increases following acutely induced cardiac hypertrophy were investigated in mature Sprague-Dawley rats. Special attention was given to the cellular organization of the growth process. Autoradiographic studies provided conclusive evidence that the uptake of tritiated thymidine is completely limited to nonmuscular cellular elements, chiefly connective tissue cells. The frequency of labeled nuclei was increased by sixfold during hypertrophy. The thymidine pool was not appreciably different in the hypertrophied hearts. Connective tissue nuclei formed a larger proportion of the total nuclear population in hypertrophied hearts, and their distribution was less uniform than in the normal heart. Quantitative histologic studies also showed that the total number of left ventricular muscle cell nuclei did not increase during hypertrophy but rather may have decreased slightly. Both the concentration and the total amount of hydroxyproline increased in parallel with the proliferative changes in the connective tissue and provide further supportive evidence to the autoradiographic and histologic studies