847 research outputs found
Computational Approaches to Consecutive Pattern Avoidance in Permutations
In recent years, there has been increasing interest in consecutive pattern
avoidance in permutations. In this paper, we introduce two approaches to
counting permutations that avoid a set of prescribed patterns consecutively.
These algoritms have been implemented in the accompanying Maple package CAV,
which can be downloaded from the author's website. As a byproduct of the first
algorithm, we have a theorem giving a sufficient condition for when two pattern
sets are strongly (consecutively) Wilf-Equivalent. For the implementation of
the second algorithm, we define the cluster tail generating function and show
that it always satisfies a certain functional equation. We also explain how the
CAV package can be used to approximate asymptotic constants for single pattern
avoidance.Comment: 12 page
On the Asymptotic Statistics of the Number of Occurrences of Multiple Permutation Patterns
We study statistical properties of the random variables ,
the number of occurrences of the pattern in the permutation . We
present two contrasting approaches to this problem: traditional probability
theory and the ``less traditional'' computational approach. Through the
perspective of the first one, we prove that for any pair of patterns
and , the random variables and are jointly
asymptotically normal (when the permutation is chosen from ). From the
other perspective, we develop algorithms that can show asymptotic normality and
joint asymptotic normality (up to a point) and derive explicit formulas for
quite a few moments and mixed moments empirically, yet rigorously. The
computational approach can also be extended to the case where permutations are
drawn from a set of pattern avoiders to produce many empirical moments and
mixed moments. This data suggests that some random variables are not
asymptotically normal in this setting.Comment: 18 page
Automatic Generation of Theorems and Proofs on Enumerating Consecutive-Wilf classes
This article, dedicated to Herbert Saul Wilf on the occaison of his
forthcoming 80-th birthday, describes two complementary approaches to
enumeration, the "positive" and the "negative", each with its advantages and
disadvantages. Both approaches are amenable to automation, and when applied to
the currently active subarea, initiated in 2003 by Sergi Elizalde and Marc Noy,
of enumerating consecutive-Wilf classes (i.e. consecutive pattern-avoidance) in
permutations, were successfully pursued by DZ's two current PhD students,
Andrew Baxter and Brian Nakamura. The Maple packages SERGI and ELIZALDE,
implementing the algorithms enable the computer to "do research" by deriving,
"all by itself", functional equations for the generating functions that enable
polynomial-time enumeration for any set of patterns. In the case of ELIZALDE
(the "negative" approach), these functional equations can be sometimes
(automatically!) simplified, and imply "explicit" formulas, that previously
were derived by humans using ad-hoc methods. We also get lots of new "explicit"
results, beyond the scope of humans, but we have to admit, that we still need
humans to handle "infinite families" of patterns, but this too, no doubt, will
soon be automatable, and we leave it as a challenge to the (human and/or
computer) reader. The Maple packages, and lots of sample output, is available
from the webpage of this article:
http://www.math.rutgers.edu/~zeilberg/mamarim/mamarimhtml/auto.htmlComment: 17 page
The Biermann Battery in Cosmological MHD Simulations of Population III Star Formation
We report the results of the first self-consistent three-dimensional adaptive
mesh refinement magnetohydrodynamical simulations of Population III star
formation including the Biermann Battery effect. We find that the Population
III stars formed including this effect are both qualitatively and
quantitatively similar to those from hydrodynamics-only (non-MHD) cosmological
simulations. We observe peak magnetic fields of ~10^-9 G in the center of our
star-forming halo at z ~ 17.55. The magnetic fields created by the Biermann
Battery effect are predominantly formed early in the evolution of the
primordial halo at low density and large spatial scales, and then grow through
compression and by shear flows. The fields seen in this calculation are never
large enough to be dynamically important (with beta >= 10^{15} at all times),
and should be considered the minimum possible fields in existence during
Population III star formation, and may be seed fields for the stellar dynamo or
the magnetorotational instability at higher densities and smaller spatial
scales.Comment: 4 pages, 3 figures; Submitted to the Astrophysical Journal Letters.
Comments welcome. Typo found (and fixed) in equation
Recommended from our members
Effects of irrigation frequency on yields of winter wheat
Deliberately underirrigating a crop may yield economic
benefits. The optimal use of water by an irrigator
should be achieved under a deficit irrigation regime. It
is important to know how water deficits affect yields and
the interaction of the deficits with the scheduling of
irrigations.
A field experiment was conducted during the 1981 irrigation
season to investigate the effects of high and low
frequency deficit irrigation on yields of winter wheat.
Yield and water use data were used to construct three production
functions. The relationship between the level of
water use and the resulting yield were determined for three
irrigation frequency regimes. Field plots under daily
(high frequency), weekly (normal frequency), and stress
(reduced frequency) regimes were included in the field
experiment as well as two dryland production plots.
The relationships derived from this project were characterized
by a large degree of scatter in the results.
Highly favorable weather conditions offset the effects of
irrigation deficits on plant yields throughout the irrigation
season. At this time, a second year of data is in the
process of compilation.
The results of a regression analysis showed no statistically
significant difference in the water use-yield
relationships of the three irrigation frequency regimes.
The effect of frequency did not lead to any readily apparent
differences in the three production functions.
The efficiency of water use of the different irrigation
frequencies increases with decreasing amount of applied
water for the 1981 crop year. The most efficient treatment,
i.e., least water use per unit of yield, was the pre-plant
irrigated, dryfarmed plots. The density of the wheat, a
measure of crop quality also increased with decreased water
use in this experiment. The optimal irrigation treatment
(measured by production and quality) was the two-week
frequency set
Formation of the Black Hole in Nova Scorpii
Israelian et al. (1999) showed that the stellar companion of the black-hole
binary Nova Sco is polluted with material ejected in the supernova that
accompanied the formation of the black-hole primary. Here we systematically
investigate the implications of these observations for the black-hole formation
process. Using a variety of supernova models, including both standard as well
as hypernova models (for different helium-star masses, explosion energies, and
explosion geometries) and a simple model for the evolution of the binary and
the pollution of the secondary, we show that most of the observed abundance
anomalies can be explained for a large range of model parameters (apart from
the abundance of Ti). The best models are obtained for He star masses of 10 to
16 Msun, where spherical hypernova models are generally favoured over standard
supernova ones. Aspherical hypernova models also produce acceptable fits,
provided there is extensive lateral mixing. All models require substantial
fallback and that the fallback material either reached the orbit of the
secondary or was mixed efficiently with material that escaped. The black hole
therefore formed in a two-step process, where the initial mass of the collapsed
remnant was increased substantially by matter that fell back after the initial
collapse. This may help to explain the high observed space velocity of Nova Sco
either because of a neutrino-induced kick (if a neutron star was formed first)
or by asymmetric mass ejection in an asymmetric supernova explosion.Comment: 16 pages, 3 Figures, 4 Tables. submitted to Ap
The basic ingredients of the North Atlantic storm track. Part I: land-sea contrast and orography
Understanding and predicting changes in storm tracks over longer time scales is a challenging problem, particularly in the North Atlantic. This is due in part to the complex range of forcings (land–sea contrast, orography, sea surface temperatures, etc.) that combine to produce the structure of the storm track. The impact of land–sea contrast and midlatitude orography on the North Atlantic storm track is investigated through a hierarchy of GCM simulations using idealized and “semirealistic” boundary conditions in a high-resolution version of the Hadley Centre atmosphere model (HadAM3). This framework captures the large-scale essence of features such as the North and South American continents, Eurasia, and the Rocky Mountains, enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies. The physical processes by which the forcing mechanisms impact the large-scale flow and the midlatitude storm tracks are discussed. The characteristics of the North American continent are found to be very important in generating the structure of the North Atlantic storm track. In particular, the southwest–northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the Rocky Mountains leads to enhanced storm development along an axis close to that of the continent’s eastern coastline. The approximately triangular shape of North America also enables a cold pool of air to develop in the northeast, intensifying the surface temperature contrast across the eastern coastline, consistent with further enhancements of baroclinicity and storm growth along the same axis
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