559 research outputs found
Extremal Graphs for Intersecting Triangles
AbstractIt is known that for a graph on n vertices [n2/4] + 1 edges is sufficient for the existence of many triangles. In this paper, we determine the minimum number of edges sufficient for the existence of k triangles intersecting in exactly one common vertex
The impact of parent-created motivational climate on adolescent athletes' perceptions of physical self-concept
This is a preliminary version of this article. The official published version can be obtained from the link below.Grounded in expectancy-value model (Eccles, 1993) and achievement goal theory (Nicholls, 1989), this study examined the perceived parental climate and its impact on athletes' perceptions of competence and ability. Hierarchical regression analyses with a sample of 237 British adolescent athletes revealed that mothers and fathers' task- and ego-involving climate predicted their son's physical self-concept; the father in particular is the strongest influence in shaping a son's physical self-concept positively and negatively. It was also found that the self-concept of the young adolescent athlete is more strongly affected by the perceived parental-created motivational climate (both task and ego) than the older adolescent athlete's self-concept. These findings support the expectancy-value model assumptions related to the role of parents as important socializing agents, the existence of gender-stereotyping, and the heavy reliance younger children place on parents' feedback
Non-linear screening corrections of stellar nuclear reaction rates and their effects on solar neutrino fluxes
Non-linear electron screening corrections of stellar nuclear fusion rates are
calculated analytically in the framework of the Debye-Huckel model and compared
with the respective ones of Salpeter's weak screening approximation. In typical
solar conditions, the deviation from Salpeter's screening factor is less than
one percent, while for hotter stars such corrections turn out to be of the
order of one percent only over the limits of the Debye-Huckel model. Moreover,
an investigation of the impact of the derived non-linear screening effects on
the solar neutrino fluxes yields insignificant corrections for both the pp and
CNO chain reactions.Comment: To appear in Phys.Rev.
Hamiltonicity of 3-arc graphs
An arc of a graph is an oriented edge and a 3-arc is a 4-tuple of
vertices such that both and are paths of length two. The
3-arc graph of a graph is defined to have vertices the arcs of such
that two arcs are adjacent if and only if is a 3-arc of
. In this paper we prove that any connected 3-arc graph is Hamiltonian, and
all iterative 3-arc graphs of any connected graph of minimum degree at least
three are Hamiltonian. As a consequence we obtain that if a vertex-transitive
graph is isomorphic to the 3-arc graph of a connected arc-transitive graph of
degree at least three, then it is Hamiltonian. This confirms the well known
conjecture, that all vertex-transitive graphs with finitely many exceptions are
Hamiltonian, for a large family of vertex-transitive graphs. We also prove that
if a graph with at least four vertices is Hamilton-connected, then so are its
iterative 3-arc graphs.Comment: in press Graphs and Combinatorics, 201
A Search for Ultra-High Energy Counterparts to Gamma-Ray Bursts
A small air shower array operating over many years has been used to search
for ultra-high energy (UHE) gamma radiation ( TeV) associated with
gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton
Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each
burst are presented for seven GRBs located with zenith angles . A excess over background was observed between 10 and
20 minutes following the onset of a GRB on 11 May 1991. The confidence level
that this is due to a real effect and not a background fluctuation is 99.8\%.
If this effect is real then cosmological models are excluded for this burst
because of absorption of UHE gamma rays by the intergalactic radiation fields.Comment: 4 pages LaTeX with one postscript figure. This version does not use
kluwer.sty and will allow automatic postscript generatio
Diffusion-limited reactions and mortal random walkers in confined geometries
Motivated by the diffusion-reaction kinetics on interstellar dust grains, we
study a first-passage problem of mortal random walkers in a confined
two-dimensional geometry. We provide an exact expression for the encounter
probability of two walkers, which is evaluated in limiting cases and checked
against extensive kinetic Monte Carlo simulations. We analyze the continuum
limit which is approached very slowly, with corrections that vanish
logarithmically with the lattice size. We then examine the influence of the
shape of the lattice on the first-passage probability, where we focus on the
aspect ratio dependence: Distorting the lattice always reduces the encounter
probability of two walkers and can exhibit a crossover to the behavior of a
genuinely one-dimensional random walk. The nature of this transition is also
explained qualitatively.Comment: 18 pages, 16 figure
QCD Corrections to QED Vacuum Polarization
We compute QCD corrections to QED calculations for vacuum polarization in
background magnetic fields. Formally, the diagram for virtual loops
is identical to the one for virtual loops. However due to
confinement, or to the growth of as decreases, a direct
calculation of the diagram is not allowed. At large we consider the
virtual diagram, in the intermediate region we discuss the role of
the contribution of quark condensates \left and at the
low-energy limit we consider the , as well as charged pion
loops. Although these effects seem to be out of the measurement accuracy of
photon-photon laboratory experiments they may be relevant for -ray
burst propagation. In particular, for emissions from the center of the galaxy
(8.5 kpc), we show that the mixing between the neutral pseudo-scalar pion
and photons renders a deviation from the power-law spectrum in the
range. As for scalar quark condensates \left and
virtual loops are relevant only for very high radiation density
and very strong magnetic fields of order .Comment: 15 pages, 4 figures; Final versio
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
Gamma Ray Bursts as Probes of Quantum Gravity
Gamma ray bursts (GRBs) are short and intense pulses of -rays
arriving from random directions in the sky. Several years ago Amelino-Camelia
et al. pointed out that a comparison of time of arrival of photons at different
energies from a GRB could be used to measure (or obtain a limit on) possible
deviations from a constant speed of light at high photons energies. I review
here our current understanding of GRBs and reconsider the possibility of
performing these observations.Comment: Lectures given at the 40th winter school of theretical physics:
Quantum Gravity and Phenomenology, Feb. 2004 Polan
Atomic diffraction from nanostructured optical potentials
We develop a versatile theoretical approach to the study of cold-atom
diffractive scattering from light-field gratings by combining calculations of
the optical near-field, generated by evanescent waves close to the surface of
periodic nanostructured arrays, together with advanced atom wavepacket
propagation on this optical potential.Comment: 8 figures, 10 pages, submitted to Phys. Rev.
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