1,251 research outputs found

    Modular Decomposition and the Reconstruction Conjecture

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    We prove that a large family of graphs which are decomposable with respect to the modular decomposition can be reconstructed from their collection of vertex-deleted subgraphs.Comment: 9 pages, 2 figure

    Description of economic data collected with a random sample of commercial reef fish boats in the Florida Keys

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    This study summarizes the results of a survey designed to provide economic information about the financial status of commercial reef fish boats with homeports in the Florida Keys. A survey questionnaire was administered in the summer and fall of 1994 by interviewers in face-to-face meetings with owners or operators of randomly selected boats. Fishermen were asked for background information about themselves and their boats, their capital investments in boats and equipment, and about their average catches, revenues, and costs per trip for their two most important kinds of fishing trips during 1993 for species in the reef fish fishery. Respondents were characterized with regard to their dependence on the reef fish fishery as a source of household income. Boats were described in terms of their physical and financial characteristics. Different kinds of fishing trips were identified by the species that generated the greatest revenue. Trips were grouped into the following categories: yellowtail snapper (Ocyurus chrysurus); mutton snapper (Lutjanus analis), black grouper (Mycteroperca bonaci), or red grouper (Epinephelus morio); gray snapper (Lutjanus griseus); deeper water groupers and tilefishes; greater amberjack (Seriola dumerili); spiny lobster (Panulirus argus); king mackerel (Scomberomorus cavalla); and dolphin (Coryphaena hippurus). Average catches, revenues, routine trip costs, and net operating revenues per boat per trip and per boat per year were estimated for each category of fishing trips. In addition to its descriptive value, data collected during this study will aid in future examinations of the economic effects of various regulations on commercial reef fish fishermen.(PDF file contains 48 pages.

    Circular choosability is rational

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    AbstractThe circular choosability or circular list chromatic number of a graph is a list-version of the circular chromatic number, that was introduced by Mohar in 2002 and has been studied by several groups of authors since then. One of the nice properties that the circular chromatic number enjoys is that it is a rational number for all finite graphs G, and a fundamental question, posed by Zhu and reiterated by others, is whether the same holds for the circular choosability. In this paper we show that this is indeed the case

    Simple formulas for lattice paths avoiding certain periodic staircase boundaries

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    There is a strikingly simple classical formula for the number of lattice paths avoiding the line x = ky when k is a positive integer. We show that the natural generalization of this simple formula continues to hold when the line x = ky is replaced by certain periodic staircase boundaries--but only under special conditions. The simple formula fails in general, and it remains an open question to what extent our results can be further generalized.Comment: Accepted version (JCTA); proof of Corollary 7 expanded, and 2 new refs adde

    School District Leadership that Works: The Effect of Superintendent Leadership on Student Achievement

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    As the latest in its continuing series of research on school, leadership, and classroom practices related to student achievement, McREL conducted a meta-analysis of research on the effect of superintendent leadership on student achievement. For this study, McREL researchers identified 27 research reports conducted since 1970 that examined, using quantitative, rigorous methods, the influence of school district leaders on student performance. Using a sophisticated research technique called a meta-analysis, McREL combined data from separate studies into a single sample, creating what McREL believes to be the largest-ever quantitative examination of research on superintendents. Altogether, these studies involved 2,714 districts and the achievement scores of 3.4 million students. The study produced the following major findings. Finding 1: District-level leadership matters. The McREL research team, led by McREL President and CEO Tim Waters and McREL Senior Fellow Robert J. Marzano, found a statistically significant relationship (a positive correlation of .24) between district leadership and student achievement.Finding 2: Effective superintendents focus their efforts on creating goal-oriented districts. McREL researchers also identified five district-level leadership responsibilities that have a statistically significant correlation with average student academic achievement. All five of these responsibilities relate to setting and keeping districts focused on teaching and learning goals.Finding 3: Superintendent tenure is positively correlated with student achievement. McREL found two studies that looked specifically at the correlations between superintendent tenure and student achievement. The weighted average correlation in these two studies was a statistically significant .19, which suggests that length of superintendent tenure in a district positively correlates to student achievement. These positive effects appear to manifest themselves as early as two years into a superintendent's tenure

    Performance of Two Boron-Modified S-816 Alloys in a Turbojet Engine Operated at 1650 F

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    S-816+B and modified S-816+B cast cobalt-base alloys were evaluated as turbine-bucket materials at 16500 F. Stress-rupture and tensile data obtained from these alloys had indicated satisfactory strength for engine operation at 1650 F. Although both alloys exhibited a limited ductility in room-temperature laboratory impact tests, there was a significant increase in impact resistance in the 1650 F tests. Bucket failures began after 10 hours of engine testing and continued at various intervals during the 107.5-hour test. Bucket lives were short relative to the predicted lives based on stress-rupture considerations (280 hr for S-816+B and 1750 hr for modified S-816+B). No significant difference was apparent in the performance of the two alloy groups. The primary cause of bucket failures in both alloys was mechanical fatigue. Impact damage occurred as a direct result of bucket tip fatigue failures and was a secondary cause of bucket failures. The impact of small pieces of fractured bucket tips on surrounding buckets caused a relatively large amount of impact damage to buckets of both alloys. The amount of impact damage from induced fractures at the bucket midspan, which provided relatively large failed fragments, was no greater than that which occurred as a result of tip failures

    Modular decomposition and the Reconstruction Conjecture

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    Abstract We prove that a large family of graphs which are decomposable with respect to the modular decomposition can be reconstructed from their collection of vertex-deleted subgraphs

    Mathematical modelling of fibre-enhanced perfusion inside\ud a tissue-engineering bioreactor

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    We develop a simple mathematical model for forced flow of culture medium through a porous scaffold in a tissue- engineering bioreactor. Porous-walled hollow fibres penetrate the scaffold and act as additional sources of culture medium. The model, based on Darcy’s law, is used to examine the nutrient and shear-stress distributions throughout the scaffold. We consider several configurations of fibres and inlet and outlet pipes. Compared with a numerical solution of the full Navier–Stokes equations within the complex scaffold geometry, the modelling approach is cheap, and does not require knowledge of the detailed microstructure of the particular scaffold being used. The potential of this approach is demonstrated through quantification of the effect the additional flow from the fibres has on the nutrient and shear-stress distribution
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