7,328 research outputs found
Even Orientations and Pfaffian graphs
We give a characterization of Pfaffian graphs in terms of even orientations,
extending the characterization of near bipartite non--pfaffian graphs by
Fischer and Little \cite{FL}. Our graph theoretical characterization is
equivalent to the one proved by Little in \cite{L73} (cf. \cite{LR}) using
linear algebra arguments
Even Orientations of Graphs: Part I
A graph G is 1-extendable if every edge belongs to at least one 1-factor. Let
G be a graph with a 1-factor F. Then an even F-orientation of G is an
orientation in which each F-alternating cycle has exactly an even number of
edges directed in the same fixed direction around the cycle.
In this paper, we examine the structure of 1-extendible graphs G which have
no even F-orientation where F is a fixed 1-factor of G. In the case of cubic
graphs we give a characterization. In a companion paper [M. Abreu, D. Labbate
and J. Sheehan. Even orientations of graphs: Part II], we complete this
characterization in the case of regular graphs, graphs of connectivity at least
four and k--regular graphs for . Moreover, we will point out a
relationship between our results on even orientations and Pfaffian graphs
developed in [M. Abreu, D. Labbate and J. Sheehan. Even orientations and
Pfaffian graphs].Comment: 40 pages, 2 figure
J D Bernal: philosophy, politics and the science of science
This paper is an examination of the philosophical and political legacy of John Desmond Bernal. It addresses the evidence of an emerging consensus on Bernal based on the recent biography of Bernal by Andrew Brown and the reviews it has received. It takes issue with this view of Bernal, which tends to be admiring of his scientific contribution, bemused by his sexuality, condescending to his philosophy and hostile to his politics. This article is a critical defence of his philosophical and political position
J D Bernal: philosophy, politics and the science of science
This paper is an examination of the philosophical and political legacy of John Desmond Bernal. It addresses the evidence of an emerging consensus on Bernal based on the recent biography of Bernal by Andrew Brown and the reviews it has received. It takes issue with this view of Bernal, which tends to be admiring of his scientific contribution, bemused by his sexuality, condescending to his philosophy and hostile to his politics. This article is a critical defence of his philosophical and political position
Were all extinction events caused by impacts?
Extraterrestrial impacts are firmly implicated in several of the five major Phanerozoic extinction events. A critical issue now is whether extraterrestrial events have been the only mechanism that produced physical changes of sufficient magnitude to cause major extinction events. While we believe the evidence is overwhelming that the KT extinction event was caused by an impact, we also find that an event of similar or larger size near the end of the Ordovician is best explained by terrestrial causes. The Ordovician extinction event (End-O extinction event) occurred near the end of the Ordovician, but the interval of extinction was completed prior to the newly established Ordovician-Silurian boundary. In spite of extensive field studies, a convincing signature of an associated impact has not been found. However, a prominent glaciation does coincide with the End-O extinction event
‘Deliberate Preparation’ as an evidence-based focus for primary physical education
There is substantial scientific research suggesting the physical and psychological health benefits of a physically active lifestyle. Consequently, governments worldwide prioritize policies, finances, and resources in healthcare, education, and sports sectors to increase mass participation in physical activity. However, practices in physical activity promotion are often not underpinned by evidence-based standardization that is requisite in other domains of epidemiology. The aim of this article is to examine critically the available scientific research on promoting life-long physical activity participation and to propose an evidence-based model for implementation in school physical education. Reasons are discussed as to why programs that integrate physical, psychological, and behavioral skills have been long acknowledged in physical education and physical activity domains but remain lacking in empirical validation. Finally, future directions are suggested that are required to examine the application of this approach to practice in primary-level physical education
Evolution of Mass Outflow in Protostars
We have surveyed 84 Class 0, Class I, and flat-spectrum protostars in
mid-infrared [Si II], [Fe II] and [S I] line emission, and 11 of these in
far-infrared [O I] emission. We use the results to derive their mass outflow
rates. Thereby we observe a strong correlation of mass outflow rates with
bolometric luminosity, and with the inferred mass accretion rates of the
central objects, which continues through the Class 0 range the trend observed
in Class II young stellar objects. Along this trend from large to small
mass-flow rates, the different classes of young stellar objects lie in the
sequence Class 0 -- Class I/flat-spectrum -- Class II, indicating that the
trend is an evolutionary sequence in which mass outflow and accretion rates
decrease together with increasing age, while maintaining rough proportionality.
The survey results include two which are key tests of magnetocentrifugal
outflow-acceleration mechanisms: the distribution of the outflow/accretion
branching ratio b, and limits on the distribution of outflow speeds. Neither
rule out any of the three leading outflow-acceleration,
angular-momentum-ejection mechanisms, but they provide some evidence that disk
winds and accretion-powered stellar winds (APSWs) operate in many protostars.
An upper edge observed in the branching-ratio distribution is consistent with
the upper bound of b = 0.6 found in models of APSWs, and a large fraction
(0.31) of the sample have branching ratio sufficiently small that only disk
winds, launched on scales as large as several AU, have been demonstrated to
account for them.Comment: Version submitted to ApJ: 36 pages, 3 tables, 8 figure
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