458 research outputs found
Correspondence to Mary Ann Smith From William H. Borders and C.M. Lowe, March 22, 1961
No full textCorrespondence from William H. Borders and C.M. Lowe to Mary Ann Smith notifying her of a meeting for the Atlanta Student Adult Liaison. 1 page
Learning to simulate and predict chaotic dynamical systems
No full textWith precise knowledge of the rules which govern a deterministic chaotic system, it is possible to interact with the system and change its dynamics. This research is part of a larger project, in which chaos control is used to improve the bubbling behavior of multi-phase chemical reactors. Chaos control requires models which capture the complete behavior of the system. If we replace the system by its model, or vice versa, we should not notice a change in dynamical behavior. We restrict ourselves to data-driven models, which learn both their structure and parameters from measured data. In cooperation with Robert Jan de Korte [1], we use a neural network model to control the chaotic dynamics of an experimental, driven and damped pendulum. The neural network provides a nearly perfect model for this system. The gas-solids fluidized bed is a much more difficult system, because it has a large number of state variables, while the pendulum has only three. To get a good predictive model, the neural network approach is improved with several enhancements: (1) Inputs are compressed by weighted Principal Component Analysis. (2) An 'error propagation" scheme is introduced, in the which the model synchronizes itself with the data. (3) The neural network is connected in parallel to a linear predictive model. (4) A new pruning algorithm removes unused nodes from the network. (5) A statistical test by Diks et. al. [3] compares the chaotic attractors of the model-generated and measured time series. The approach is succesfully applied to benchmark tests. But Diks' test reveals that during training, the correctness of the model's attractor jumps from right to wrong from one iteration to another. We investigate why this is, and present an example of a model which can predict the measured data with zero error, but yet has a very different attractor. This has far-reaching consequences. It turns out that the learning of an attractor from measured data is a very ill-posed problem, in which the data covers all available dimensions globally, but locally the data is confined to low-dimensional structures. When a global nonlinear model is trained on this data, it locally has too many degrees of freedom, and this leads to arbitrary dynamics. A Nonlinear Principal Component Regression (NLPCR) algorithm is needed, which locally detects and eliminates the unused dimensions. We develop the 'Split & Fit' (S&F) algorithm, based on a fuzzy partitioning of the input space. In each region, unused dimensions are detected with Principal Component Analysis (PCA). This algorithm is shown to keep an otherwise unstable model for a chaotic laser onto the desired trajectory. Meanwhile, Robert Jan de Korte found that deterministic prediction of gas-solids fluidized beds is not feasible. But the S&F algorithm does learn the attractor of another experimental reactor, a gas-liquid bubble column with a single train of rising bubbles [2]. The S&F model paves the way for robust learning of chaotic attractors. However, real-world systems rarely meet the requirement of determinism and low-dimensionality. For these systems, we recommend to develop algorithms which find structure in 'noisy' nonlinear behavior. A good starting point is to have a probabilistic representation (kernel smoother or mixture density) of how the measured data are distributed in state space. [1] R.J. De Korte (2000), "Controlling the Chaotic Hydrodynamics of Fluidized Beds", PhD thesis, Delft Unversity of Technology [2] S. Kaart (2002), "Controlling Chaotic Bubbles", PhD thesis, Delft Unversity of Technology [3] C. Diks, W.R. van Zwet, F. Takens, J. de Goede (1996), "Detecting differences between delay vector distributions, pp. 2169--2176Applied Science
Connecting dynamic vegetation models to data - an inverse perspective
No full textDynamic vegetation models provide process-based explanations of the dynamics and the distribution of plant ecosystems. They offer significant advantages over static, correlative modelling approaches, particularly for ecosystems that are outside their equilibrium due to global change or climate change. A persistent problem, however, is their parameterization. Parameters and processes of dynamic vegetation models (DVMs) are traditionally determined independently of the model, while model outputs are compared to empirical data for validation and informal model comparison only. But field data for such independent estimates of parameters and processes are often difficult to obtain, and the desire to include better descriptions of processes such as biotic interactions, dispersal, phenotypic plasticity and evolution in future vegetation models aggravates limitations related to the current parameterization paradigm. In this paper, we discuss the use of Bayesian methods to bridge this gap. We explain how Bayesian methods allow direct estimates of parameters and processes, encoded in prior distributions, to be combined with inverse estimates, encoded in likelihood functions. The combination of direct and inverse estimation of parameters and processes allows a much wider range of vegetation data to be used simultaneously, including vegetation inventories, species traits, species distributions, remote sensing, eddy flux measurements and palaeorecords. The possible reduction of uncertainty regarding structure, parameters and predictions of DVMs may not only foster scientific progress, but will also increase the relevance of these models for policy advice
Whole-genome analysis of diverse Chlamydia trachomatis strains identifies phylogenetic relationships masked by current clinical typing
No full textChlamydia trachomatis is responsible for both trachoma and sexually transmitted infections, causing substantial morbidity and economic cost globally. Despite this, our knowledge of its population and evolutionary genetics is limited. Here we present a detailed phylogeny based on whole-genome sequencing of representative strains of C. trachomatis from both trachoma and lymphogranuloma venereum (LGV) biovars from temporally and geographically diverse sources. Our analysis shows that predicting phylogenetic structure using ompA, which is traditionally used to classify Chlamydia, is misleading because extensive recombination in this region masks any true relationships present. We show that in many instances, ompA is a chimera that can be exchanged in part or as a whole both within and between biovars. We also provide evidence for exchange of, and recombination within, the cryptic plasmid, which is another key diagnostic target. We used our phylogenetic framework to show how genetic exchange has manifested itself in ocular, urogenital and LGV C. trachomatis strains, including the epidemic LGV serotype L2b
Living stones : the practice of remembrance at Lincoln Cathedral, (1092-1235)
No full textThis thesis analyses four different aspects of devotional life at one of England’s largest and
wealthiest medieval cathedrals between the years 1092 and 1235. Each of these is associated with
the remembrance of the dead. It is an area of religious practice that was subject to momentous
change over the course of the period. These changes would have a profound effect on the
organization of Christian worship for centuries to come. The thesis assesses how contrasting
approaches to the practice of remembrance were able to enhance and shape the composition of
the church, and explores what they reveal about the distinctive fellowship of a secular cathedral
Testing for Relative Predictive Accuracy: A Critical Viewpoint
No full textTests for relative predictive accuracy have become a widespread addendum to forecast comparisons. Many empirical research reports conclude that the difference between the entertained forecasting models is 'insignificant'. This paper collects arguments that cast doubt on the usefulness of relative predictive accuracy tests. The main point is not that test power is too low but that their application is conceptually mistaken. The features are highlighted by means of some Monte Carlo experiments for simple time-series decision problems.Information criteria, Forecasting, Hypothesis testing
Book Reviews
No full textBook Review 1Book Title: Reproductive Energetics in MammalsBook Authors: A.S.I. Loudon & P.A. Racey (Eds.)Zoological Society of London Symposia 57, 1987 Clarendon. Press, Oxford. 371 pp.Book Review 2Book Title: Classification of Southern African MammalsBook Authors: J.A.J. Meester, I.L. Rautenbach, N.J. Dippenaar & C.M. BakerTransvaal Museum Monograph No.5. 359 pp.Book Review 3Book Title: Pesticide impact on stream fauna with special reference to macroinvertebratesBook Author: R.C. Muirhead-ThomsonCambridge University Press, 1987. 275 pp.Book Review 4Book Title: Evolution of sex determining mechanismsBook Author: James J. BullBenjamin-Cummings Publ. Company / Addison-Wesley Publishing Group, JohannesburgBook Review 5Book Title: The evolutionary ecology of ant-plant mutualismsBook Author: Andrew J. BeattieCambridge University Press 182 pp.Book Review 6Book Title: The Ecology of SexBook Authors: P.J. Greenwood & J. Adams Edward Arnold, London, 1987. 74 pagesBook Review 7Book Title: The Dinosaur Heresies - a revolutionary view of dinosaursBook Author: Robert BakkerPublished by Longman Scientific and Technical, 1987Book Review 8Book Title: Molecular Biology of the GeneBook Authors: Watson, Hopkins, Roberts, Steitz & WeinerVolumes I and II (Fourth Edition) (Benjamin/Cummings. Menlo Park); Addison-Wesley Publishing Group. Johannesburg 1163 pp.Book Review 9Book Title: Evolutionary BiologyBook Author: Eli C. MinkoffAddison-Wesley Publishing Company, Massachussets, 1983. 627pp.Book Review 10Book Title: An ecosystem approach to aquatic ecology. Mirror Lake and its environmentBook Author: Gene E. Likens (Ed.)Springer-Ver1ag, New York. xiv - 516 pages; 197 figuresBook Review 11Book Title: The Physiological Ecology of SeaweedsBook Authors: C.S. Lobban, P.J. Harrison & M.J. Duncan Cambridge University press, Cambridge, 1985. 242 pagesBook Review 12Book Title: Principles of ecologyBook Authors: R.J.Putman & S.D. WrattenCroom Helm, London, 1984. 388 pages
A status report of Wisconsin public secondary technology education programs and their historical, comparative nature with national and state studies
No full textPlan BThe purpose of this descriptive study was to provide benchmark data regarding the status of Technology Education Programs in public secondary schools in the state of Wisconsin. This study solicited information from a representative sample of Wisconsin TE program chairpersons regarding program demographics, current practice, and program purpose/barriers. Specifically, a 65-question survey was mailed to 175 department chairpersons requesting feedback on program name, mission, facilities, gender and minority enrollment, course offerings, class sizes, faculty and student demographics, instructional methodology, curriculum preparation, course titles, program purpose, and barriers to building, maintaining, and improving upon an exemplary TE program. Survey data was compiled and subsequently analyzed utilizing SPSS software. The resulting information was then presented in two formats: a) as a conglomerate group and b) by school size (small, medium, and large cohorts). Current findings identified the characteristics of Wisconsin Technology Education programs, faculty, and students. The research examined technology education curricula, content, and instructional methods followed in the state of Wisconsin at the high school level. Furthermore, the study determined the course titles currently being taught and discussed what those titles might suggest about the status of the profession. The findings also revealed the opinions of Technology Education regarding purpose and impediments to exemplary curriculum. Finally the research compared the current status of Wisconsin Technology Education programs with those of the industrial arts/technology education programs of the 1960s, and 1990s at state and national levels. The current research was significant in that it provided fact-based evidence concerning the current condition, respective trends, and comparative nature to previous findings regarding Wisconsin Technology Education programs to a wide range of professionals. The Department of Public Instruction and higher education institutions may utilize the data to warrant development of state-wide initiatives, supervisory support for teachers, increased program funding, alterations in teacher preparation, and mandates for specific in-service training. Legislators may find the information useful when writing policy and forming budget proposals regarding the state education system. At the local level, school districts and technology education teachers can exercise this new knowledge when evaluating programs and timing the implementation of new curricula
Moderate inflation
No full textInflation persists at moderate rates (15-30 percent) in all the countries that successfully reduced triple-digit inflation in the 1980s. Several other countries--for example, Colombia--have experienced moderate inflation for prolonged periods. The authors introduce types of theories of persistent inflation. Theories emphasizing seigniorage as a source of government finance and those emphasizing the costs of ending inflation were detailed. They examine thesources and persistence of episodes of moderate inflation. Most episodes were triggered by commodity price shocks and were brief. Very few ended in higher inflation. This report presents case studies of eight countries, including three that now suffer from moderate inflation and four that successfully moved down to single-digit inflation rates. The authors analyze the roles of seigniorage, indexation and disindexation, the exchange rate commitment, and monetary and fiscal policy. The evidence suggests that seigniorage plays, at most, a modest role in the persistence of moderate inflation and that such inflation can be reduced only at a substantial short-term cost to growth.Economic Theory&Research,Environmental Economics&Policies,Inflation,Banks&Banking Reform,Economic Conditions and Volatility
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