24,088 research outputs found
Categorical Groups, Knots and Knotted Surfaces
We define a knot invariant and a 2-knot invariant from any finite categorical
group. We calculate an explicit example for the Spun Trefoil.Comment: 40 pages, lots of figures. Second version: Added example and
discussion, clarification of the fact that the maps associated with
Reidemeister Moves are well define
Comment on ``Stripes and the t-J Model''
This is a comment being submitted to Physical Review Letters on a recent
letter by Hellberg and Manousakis on stripes in the t-J model.Comment: One reference correcte
Social Effects in Science: Modelling Agents for a Better Scientific Practice
Science is a fundamental human activity and we trust its results because it
has several error-correcting mechanisms. Its is subject to experimental tests
that are replicated by independent parts. Given the huge amount of information
available, scientists have to rely on the reports of others. This makes it
possible for social effects to influence the scientific community. Here, an
Opinion Dynamics agent model is proposed to describe this situation. The
influence of Nature through experiments is described as an external field that
acts on the experimental agents. We will see that the retirement of old
scientists can be fundamental in the acceptance of a new theory. We will also
investigate the interplay between social influence and observations. This will
allow us to gain insight in the problem of when social effects can have
negligible effects in the conclusions of a scientific community and when we
should worry about them.Comment: 14 pages, 5 figure
Large-Scale MDO of a Small Satellite using a Novel Framework for the Solution of Coupled Systems and their Derivatives
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106515/1/AIAA2013-1599.pd
Wetting and drying numerical treatments for the Roe Riemann scheme
This is the author accepted manuscript. The final version is available from Taylor & Francis via the DOI in this recordAccurate characterization of wetting-drying fronts in free surface flows is challenging because it is difficult and computationally demanding to track the exact position of the interface. This work presents a novel numerical treatment of the wetting-drying fronts applied to an approximate Roe Riemann solver and compares it to four other approaches. The numerical treatments were implemented both for the shallow water equations and for the local inertial equations. The results of this comparison overall showed a good agreement. For the tests conducted it was verified that element removal numerical treatments with global distributing of water can introduce errors and degenerate the solution introducing or displacing water upstream. Local correction and flux restricting numerical treatments show the best results. The negative depth numerical treatments provided similar results to the local correction and flux restricting numerical treatment, although with mass conservation errors.This research was funded by the FCT (Fundação para a Ciência e a Tecnologia) through the doctoral grant [SFRH/BD/81869/2011] financed through the POPH/FSE (Programa Operacional Potencial Humano/Fundo Social Europeu) and the strategic project UID/MAR/04292/2013 granted by FCT to MARE
Off-axis retrieval of orbital angular momentum of light stored in cold atoms
We report on the storage of orbital angu- lar momentum (OAM) of light of a
Laguerre-Gaussian mode in an ensemble of cold cesium atoms and its re- trieval
along an axis different from the incident light beam. We employed a
time-delayed four-wave mixing configuration to demonstrate that at small angle
(2o), after storage, the retrieved beam carries the same OAM as the one encoded
in the input beam. A calculation based on mode decomposition of the retrieved
beam over the Laguerre-Gaussian basis is in agreement with the experimental
observations done at small angle values. However, the calculation shows that
the OAM retrieving would get lost at larger angles, reducing the fidelity of
such storing-retrieving process. In addition, we have also observed that by
applying an external magnetic field to the atomic ensemble the retrieved OAM
presents Larmor oscillations, demonstrating the possibility of its manipulation
and off-axis retrieval.Comment: 9 pages, 4 figure
Emergence of Hierarchy on a Network of Complementary Agents
Complementarity is one of the main features underlying the interactions in
biological and biochemical systems. Inspired by those systems we propose a
model for the dynamical evolution of a system composed by agents that interact
due to their complementary attributes rather than their similarities. Each
agent is represented by a bit-string and has an activity associated to it; the
coupling among complementary peers depends on their activity. The connectivity
of the system changes in time respecting the constraint of complementarity. We
observe the formation of a network of active agents whose stability depends on
the rate at which activity diffuses in the system. The model exhibits a
non-equilibrium phase transition between the ordered phase, where a stable
network is generated, and a disordered phase characterized by the absence of
correlation among the agents. The ordered phase exhibits multi-modal
distributions of connectivity and activity, indicating a hierarchy of
interaction among different populations characterized by different degrees of
activity. This model may be used to study the hierarchy observed in social
organizations as well as in business and other networks.Comment: 13 pages, 4 figures, submitte
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