1,547 research outputs found
Generators for Cubic Surfaces with two Skew Lines over Finite Fields
Let S be a smooth cubic surface defined over a field K. As observed by Segre
and Manin, there is a secant and tangent process on S that generates new
K-rational points from old. It is natural to ask for the size of a minimal
generating set for S(K). In a recent paper, for fields K with at least 13
elements, Siksek showed that if S contains a skew pair of K-lines then S(K) can
be generated from one point. In this paper we prove the corresponding version
of this result for fields K having at least 4 elements, and slightly milder
results for #K=2 or 3.Comment: 10 pages. arXiv admin note: text overlap with arXiv:1012.1838 by
other author
A model for the emergence of cooperation, interdependence and structure in evolving networks
Evolution produces complex and structured networks of interacting components
in chemical, biological, and social systems. We describe a simple mathematical
model for the evolution of an idealized chemical system to study how a network
of cooperative molecular species arises and evolves to become more complex and
structured. The network is modeled by a directed weighted graph whose positive
and negative links represent `catalytic' and `inhibitory' interactions among
the molecular species, and which evolves as the least populated species
(typically those that go extinct) are replaced by new ones. A small
autocatalytic set (ACS), appearing by chance, provides the seed for the
spontaneous growth of connectivity and cooperation in the graph. A highly
structured chemical organization arises inevitably as the ACS enlarges and
percolates through the network in a short, analytically determined time scale.
This self-organization does not require the presence of self-replicating
species. The network also exhibits catastrophes over long time scales triggered
by the chance elimination of `keystone' species, followed by recoveries.Comment: 8 pages, 4 figure
Monte-Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions
The complete knowledge of the nucleon spin structure at leading twist
requires also addressing the transverse spin distribution of quarks, or
transversity, which is yet unexplored because of its chiral-odd nature.
Transversity can be best extracted from single-spin asymmetries in fully
polarized Drell-Yan processes with antiprotons, where valence contributions are
involved anyway. Alternatively, in single-polarized Drell-Yan the transversity
happens convoluted with another chiral-odd function, which is likely to be
responsible for the well known (and yet unexplained) violation of the Lam-Tung
sum rule in the corresponding unpolarized cross section. We present Monte-Carlo
simulations for the unpolarized and single-polarized Drell-Yan at different center-of-mass energies in both
configurations where the antiproton beam hits a fixed proton target or it
collides on another proton beam. The goal is to estimate the minimum number of
events needed to extract the above chiral-odd distributions from future
measurements at the HESR ring at GSI. It is important to study the feasibility
of such experiments at HESR in order to demonstrate that interesting spin
physics can be explored already using unpolarized antiprotons.Comment: Deeply revised text with improved discussion of kinematics and
results; added one table; 12 figures. Accepted for publication in Phys. Rev.
q-breathers in Discrete Nonlinear Schroedinger lattices
-breathers are exact time-periodic solutions of extended nonlinear systems
continued from the normal modes of the corresponding linearized system. They
are localized in the space of normal modes. The existence of these solutions in
a weakly anharmonic atomic chain explained essential features of the
Fermi-Pasta-Ulam (FPU) paradox. We study -breathers in one- two- and
three-dimensional discrete nonlinear Sch\"{o}dinger (DNLS) lattices --
theoretical playgrounds for light propagation in nonlinear optical waveguide
networks, and the dynamics of cold atoms in optical lattices. We prove the
existence of these solutions for weak nonlinearity. We find that the
localization of -breathers is controlled by a single parameter which depends
on the norm density, nonlinearity strength and seed wave vector. At a critical
value of that parameter -breathers delocalize via resonances, signaling a
breakdown of the normal mode picture and a transition into strong mode-mode
interaction regime. In particular this breakdown takes place at one of the
edges of the normal mode spectrum, and in a singular way also in the center of
that spectrum. A stability analysis of -breathers supplements these
findings. For three-dimensional lattices, we find -breather vortices, which
violate time reversal symmetry and generate a vortex ring flow of energy in
normal mode space.Comment: 19 pages, 9 figure
Pfaffian representations of cubic surfaces
Let K be a field of characteristic zero. We describe an algorithm which
requires a homogeneous polynomial F of degree three in K[x_0,x_1,x_2,x_3] and a
zero A of F in P^3_K and ensures a linear pfaffian representation of V(F) with
entries in K[x_0,x_1,x_2,x_3], under mild assumptions on F and A. We use this
result to give an explicit construction of (and to prove the existence of) a
linear pfaffian representation of V(F), with entries in K'[x_0,x_1,x_2,x_3],
being K' an algebraic extension of K of degree at most six. An explicit example
of such a construction is given.Comment: 17 pages. Expanded with some remarks. Published with minor
corrections in Geom. Dedicat
On Binary Matroid Minors and Applications to Data Storage over Small Fields
Locally repairable codes for distributed storage systems have gained a lot of
interest recently, and various constructions can be found in the literature.
However, most of the constructions result in either large field sizes and hence
too high computational complexity for practical implementation, or in low rates
translating into waste of the available storage space. In this paper we address
this issue by developing theory towards code existence and design over a given
field. This is done via exploiting recently established connections between
linear locally repairable codes and matroids, and using matroid-theoretic
characterisations of linearity over small fields. In particular, nonexistence
can be shown by finding certain forbidden uniform minors within the lattice of
cyclic flats. It is shown that the lattice of cyclic flats of binary matroids
have additional structure that significantly restricts the possible locality
properties of -linear storage codes. Moreover, a collection of
criteria for detecting uniform minors from the lattice of cyclic flats of a
given matroid is given, which is interesting in its own right.Comment: 14 pages, 2 figure
Free-energy transition in a gas of non-interacting nonlinear wave-particles
We investigate the dynamics of a gas of non-interacting particle-like soliton
waves, demonstrating that phase transitions originate from their collective
behavior. This is predicted by solving exactly the nonlinear equations and by
employing methods of the statistical mechanics of chaos. In particular, we show
that a suitable free energy undergoes a metamorphosis as the input excitation
is increased, thereby developing a first order phase transition whose
measurable manifestation is the formation of shock waves. This demonstrates
that even the simplest phase-space dynamics, involving independent (uncoupled)
degrees of freedom, can sustain critical phenomena.Comment: 4 pages, 3 figure
Infecção congênita pelo citomegalovírus: ocorrência em duas populações de nível sócio-econômico diferentes em São Paulo, Brasil
In São Paulo, Brazil, between November 1980 and July 1982, 1614 newborns of middle socioeconomic background and 1156 newborns of low socioeconomic background were examined for the occurrence of congenital cytomegalovirus (CMV) infection by isolation of virus from urine samples or detection of specific anti-CMV IgM in umbilical cord serum tested by immunofluorescence. In the low socioeconomic population prevalence of CMV complement-fixing antibodies in mothers was 84.4%(151/179) and the incidence of congenital infection assessed by virus isolation 0.98% (5/508), as compared with 0.46% (3/648) in the group of newborns tested by detection of specific anti-CMV IgM in umbilical cord-serum. In middle socioeconomic level population prevalence of CMV complement-fixing antibodies in mothers was 66.5% (284/427) and the incidence of CMV congenital infection was 0.39% (2/518) in the group of newborns screened by virus isolation and 0.18% (2/1096) in the group tested by detection of specific anti-CMV IgM. In the present study none of the 12 congenitally infected newborns presented clinical apparent disease at birth.Entre novembro de 1980 e julho de 1982, 1614 recém-nascidos (RNs) de nivel sócio-econômico médio e 1156 RNs de baixo nível sócioeconômico foram examinados para verificar a ocorrência de infecção congênita pelo citomegalovírus (CMV), através de isolamento do vírus em amostras de urina ou detecção de anticorpos IgM específicos em amostras de sangue de cordão umbilical. Na população de baixo nível sócio econômico a prevalência de anticorpos fixadores do complemento (Ac Fc) anti-CMV nas mães foi de 84,4% (151/179) e a incidência de infecção congênita determinada por isolamento do vírus foi de 0,90% (5/508). No grupo de RNs em que o diagnóstico baseou-se apenas na detecção de Ac IgM CMV-específicos no sangue de cordão a incidência de infecção congênita foi de apenas 0,46% (3/648). Na população de nivel sócio-econômico médio a prevalência de Ac Fc anti-CMV nas mães foi de 66,5% (284/427) e a incidência de infecção congênita foi de 0,39% (2/518) no grupo de RNs testados por isolamento de vírus na urina e 0,18% (2/1090) no grupo testado por detecção de Ac IgM específicos. No presente estudo nenhum dos 12 RNs infectados congenitamente apresentou sinais ou sintomas de doença ao nascimento
Variety of idempotents in nonassociative algebras
In this paper, we study the variety of all nonassociative (NA) algebras from
the idempotent point of view. We are interested, in particular, in the spectral
properties of idempotents when algebra is generic, i.e. idempotents are in
general position. Our main result states that in this case, there exist at
least nontrivial obstructions (syzygies) on the Peirce spectrum of a
generic NA algebra of dimension . We also discuss the exceptionality of the
eigenvalue which appears in the spectrum of idempotents in
many classical examples of NA algebras and characterize its extremal properties
in metrised algebras.Comment: 27 pages, 1 figure, submitte
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