8,582 research outputs found
Hamiltonian formulation of systems with linear velocities within Riemann-Liouville fractional derivatives
The link between the treatments of constrained systems with fractional
derivatives by using both Hamiltonian and Lagrangian formulations is studied.
It is shown that both treatments for systems with linear velocities are
equivalent.Comment: 10 page
The graphene sheet versus the 2DEG: a relativistic Fano spin-filter via STM and AFM tips
We explore theoretically the density of states (LDOS) probed by an STM tip of
2D systems hosting an adatom and a subsurface impurity,both capacitively
coupled to AFM tips and traversed by antiparallel magnetic fields. Two kinds of
setups are analyzed, a monolayer of graphene and a two-dimensional electron gas
(2DEG). The AFM tips set the impurity levels at the Fermi energy, where two
contrasting behaviors emerge: the Fano factor for the graphene diverges, while
in the 2DEG it approaches zero. As result, the spin-degeneracy of the LDOS is
lifted exclusively in the graphene system, in particular for the asymmetric
regime of Fano interference. The aftermath of this limit is a counterintuitive
phenomenon, which consists of a dominant Fano factor due to the subsurface
impurity even with a stronger STM-adatom coupling. Thus we find a full
polarized conductance, achievable just by displacing vertically the position of
the STM tip. To the best knowledge, our work is the first to propose the Fano
effect as the mechanism to filter spins in graphene. This feature arises from
the massless Dirac electrons within the band structure and allows us to employ
the graphene host as a relativistic Fano spin-filter
The type N Karlhede bound is sharp
We present a family of four-dimensional Lorentzian manifolds whose invariant
classification requires the seventh covariant derivative of the curvature
tensor. The spacetimes in questions are null radiation, type N solutions on an
anti-de Sitter background. The large order of the bound is due to the fact that
these spacetimes are properly , i.e., curvature homogeneous of order 2
but non-homogeneous. This means that tetrad components of are constant, and that essential coordinates first appear as
components of . Covariant derivatives of orders 4,5,6 yield one
additional invariant each, and is needed for invariant
classification. Thus, our class proves that the bound of 7 on the order of the
covariant derivative, first established by Karlhede, is sharp. Our finding
corrects an outstanding assertion that invariant classification of
four-dimensional Lorentzian manifolds requires at most .Comment: 7 pages, typos corrected, added citation and acknowledgemen
Lagrangian formulation of classical fields within Riemann-Liouville fractional derivatives
The classical fields with fractional derivatives are investigated by using
the fractional Lagrangian formulation.The fractional Euler-Lagrange equations
were obtained and two examples were studied.Comment: 9 page
Verificação, avaliação e processamento das informações do programa de melhoramento de trigo para acesso e consulta por meio de bancos de dados relacional e grafo.
Editores técnicos: Joseani Mesquita Antunes, Ana LÃdia Variani Bonato, Márcia Barrocas Moreira Pimentel
Novel modeling formalisms and simulation tools in computational biosystems
Living organisms are complex systems that emerge
from the fundamental building blocks of life. Systems
Biology is a recent field of science that studies these
complex phenomena at the cellular level (Kitano 2002).
Understanding the mechanisms of the cell is essential
for research and development in several areas such as
drug discovery and biotechnological production. In the
latter, metabolic engineering is used for building mutant
microbial strains with increased productivity of
compounds with industrial interest, such as biofuels
(Stephanopoulos 1998). Using computational models of
cellular metabolism, it is possible to systematically test
and predict the optimal manipulations, such as gene
knockouts, that produce the ideal phenotype for a
specific application. These models are typically built in
an iterative cycle of experiment and refinement, by
multidisciplinary research teams that include biologists,
engineers and computer scientists.
The interconnection between different cellular
processes, such as metabolism and genetic regulation,
reflects the importance of the holistic approach claimed
by the Systems Biology paradigm in replacement of
traditional reductionist methods. Although most cellular
components have been studied individually, the
behavior of the cell emerges from the network-level
interaction and requires an integrative analysis. Recent
high–throughput methods have generated the so- called
omics data (e.g.: genomics, transcriptomics, proteomics,
metabolomics, fluxomics) that have allowed the
reconstruction of biological networks (Palsson 2006).
However, despite the great advances in the area, we are
still far from a whole-cell computational model that is
able to simulate all the components of a living cell. Due
to the enormous size and complexity of intracellular
biological networks, computational cell models tend to
be partial and focused on the application of interest.
Also, due to the multidisciplinarity of the field, these
models are based on several different kinds of
formalisms. Therefore, it is important to develop a
framework with common modeling formalisms, analysis
and simulation methods, that is able to accommodate
different kinds biological networks, with different types
of entities and their interactions, into genome-scale
integrated models. Cells are composed by thousands of
components that interact in myriad ways. Despite this
intricate interconnection it is usual to divide and classify
these networks according to biological function. The
main types of networks are signaling, gene regulatory
and metabolic. Signal transduction is a process for
cellular communication where the cell receives and
responds to external stimuli through signaling cascades
(Gomperts et al. 2009; Albert and Wang 2009). These
cascades affect gene regulation, which is the method for
controlling gene expression, and consequently several
cellular functions (Schlittand and Brazma 2007;
Karlebach and Sgamir 2008). Many genes encode
enzymes which are responsible for catalyzing
biochemical reactions. The complex network of these
reactions forms the cellular metabolism that sustains the
cell’s growth and energy requirements (Steuer and
Junker 2009; Palsson 2006).
The objectives of this work, in the context of a PhD
thesis, consist in re-search and selection of an
appropriate modeling formalism to develop a
framework for integration of different biological
networks, with focus on regulatory and metabolic
networks, and the implementation of suitable analysis,
simulation and optimization methods. To achieve these
goals, it is necessary to resolve many modeling issues,
such as the integration of discrete and continuous
events, representation of network topology, support for
different levels of abstraction, lack of parameters and
model complexity. This framework will be used for the
implementation of an integrated model of E. coli, a
widely used organism for industrial application
Ultrahigh energy neutrinos and non-linear QCD dynamics
The ultrahigh energy neutrino-nucleon cross sections are computed taking into
account different phenomenological implementations of the non-linear QCD
dynamic s. Based on the color dipole framework, the results for the saturation
model supplemented by DGLAP evolution as well as for the BFKL formalism in the
geometric scaling regime are presented. They are contrasted with recent
calculations using NLO DGLAP and unified BFKL-DGLAP formalisms.Comment: 5 pages, 2 figures. Version to be published in Physical Review
Type O pure radiation metrics with a cosmological constant
In this paper we complete the integration of the conformally flat pure
radiation spacetimes with a non-zero cosmological constant , and , by considering the case . This is a
further demonstration of the power and suitability of the generalised invariant
formalism (GIF) for spacetimes where only one null direction is picked out by
the Riemann tensor. For these spacetimes, the GIF picks out a second null
direction, (from the second derivative of the Riemann tensor) and once this
spinor has been identified the calculations are transferred to the simpler GHP
formalism, where the tetrad and metric are determined. The whole class of
conformally flat pure radiation spacetimes with a non-zero cosmological
constant (those found in this paper, together with those found earlier for the
case ) have a rich variety of subclasses with zero,
one, two, three, four or five Killing vectors
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