Introduction to Systems Approach

Main aspects of systems theory are outlined. Emphasis is on the interface of between time and systems - natural or artificial

Analogical acts as conceptual strategies in science, engineering and the humanities

The composing models which operate by means of analogy are identified. The importance of analogical acts in the prewriting stage of the composing process is discussed. The relations between analogical acts and concept formation are explored. A basic correspondence between the analogical thinking employed in successful learning and analogical thinking as a composing strategy is discussed. Teaching analogical acts as conceptual strategies for exploring problems and generating the form and content of discourse is presented in support of the contention that writing is a unique mode of learning

Migration signatures across the decades: Net migration by age in U.S. counties, 1950−2010

Background: Migration is the primary population redistribution process in the United States. Selective migration by age, race/ethnic group, and spatial location governs population integration, affects community and economic development, contributes to land use change, and structures service needs. Objective: Delineate historical net migration patterns by age, race/ethnic, and rural-urban dimensions for United States counties. Methods: Net migration rates by age for all US counties are aggregated from 1950−2010, summarized by rural-urban location and compared to explore differential race/ethnic patterns of age-specific net migration over time. Results: We identify distinct age-specific net migration ‘signatures’ that are consistent over time within county types, but different by rural-urban location and race/ethnic group. There is evidence of moderate population deconcentration and diminished racial segregation between 1990 and 2010. This includes a net outflow of Blacks from large urban core counties to suburban and smaller metropolitan counties, continued Hispanic deconcentration, and a slowdown in White counterurbanization. Conclusions: This paper contributes to a fuller understanding of the complex patterns of migration that have redistributed the U.S. population over the past six decades. It documents the variability in county age-specific net migration patterns both temporally and spatially, as well as the longitudinal consistency in migration signatures among county types and race/ethnic groups

Shifting Trends in Special Education

In this Fordham Institute paper, analysts examine public data and find that the national proportion of students with disabilities peaked in 2004-05 and has been declining since. This overall trend masks interesting variations; for example, proportions of students with specific learning disabilities, mental retardation, and emotional disturbances have declined, while the proportions of students with autism, developmental delays, and other health impairments have increased notably. Meanwhile, at the state level, Rhode Island, New York, and Massachusetts have the highest rates of disability identification, while Texas, Idaho, and Colorado have the lowest. The ratio of special-education teachers and paraprofessionals to special-education students also varies widely from state to state -- so much so that our analysts question the accuracy of the data reported by states to the federal government

Precise time and time interval (PTTI), an overview

Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date

Moving to Diversity

In this brief, authors Richelle Winkler and Kenneth Johnson, using new data and techniques, find that net migration between U.S. counties increased racial diversity in each of the last two decades. However, migration’s influence on diversity was far from uniform: it varied by race, age group, and location, sometimes starkly. Overall, net migration of the population under age 40 increased diversity, while net migration of people over age 60 diminished diversity. Blacks and Hispanics are migrating to predominantly white counties, while white young adults are moving to urban core counties with relatively high proportions of blacks and Hispanics. The movement of older whites is not contributing to the growing diversity, because older whites tend to move to predominantly white counties. Winkler and Johnson conclude that, while migration contributed to the growing diversity of the nation, the process was complex and varied from place to place with significant social, economic, and political implications for both the more diverse and less diverse places

Optical orientation of electron spins in GaAs quantum wells

We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 Ang and 198 Ang. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasi-thermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.Comment: 12 pages, 9 figure

Quantum black holes from null expansion operators

Using a recently developed quantization of spherically symmetric gravity coupled to a scalar field, we give a construction of null expansion operators that allow a definition of general, fully dynamical quantum black holes. These operators capture the intuitive idea that classical black holes are defined by the presence of trapped surfaces, that is surfaces from which light cannot escape outward. They thus provide a mechanism for classifying quantum states of the system into those that describe quantum black holes and those that do not. We find that quantum horizons fluctuate, confirming long-held heuristic expectations. We also give explicit examples of quantum black hole states. The work sets a framework for addressing the puzzles of black hole physics in a fully quantized dynamical setting.Comment: 5 pages, version to appear in CQ