2,929 research outputs found
Quadratic deformation of Minkowski space
We present a deformation of the Minkowski space as embedded into the
conformal space (in the formalism of twistors) based in the quantum versions of
the corresponding kinematic groups. We compute explicitly the star product,
whose Poisson bracket is quadratic. We show that the star product although
defined on the polynomials can be extended differentiably. Finally we compute
the Eucliden and Minkowskian real forms of the deformation.Comment: Presented at XVII European Workshop on String Theory 2011. Padova
(Italy) September 05-09; Fortschr. Phys. 1-7 (2012
From historical map to online 3D recreation: the 1861 cadastral map of Horta (Barcelona)
The recent study and classification of over 200 cadastral maps created in the nineteenth century in Catalonia have provided a valuable source of information about the agricultural landscape country’s past, but by linking them with data recorded in tax books known as amillaramientos, it is possible to gain a better knowledge of the past. By applying this method to the 1861 cadastral map of Horta and its corresponding amillaramiento, a planimetric map showing the land use distribution in the town was created. The resulting land use map was subsequently overlaid on top of a digital elevation model to create 3D visualizations which show the altitudinal distribution of crops and other features. Finally, the article explores a way of distributing the results online, making them accessible to the public and increasing the research impact of future findings. Therefore, the method described in this article allows the systematic recreation and distribution of past landscapes by using Catalan cadastral maps of the nineteenth century, something which can help enrich the scientific knowledge of many disciplines
Heat kernel methods for Lifshitz theories
We study the one-loop covariant effective action of Lifshitz theories using
the heat kernel technique. The characteristic feature of Lifshitz theories is
an anisotropic scaling between space and time. This is enforced by the
existence of a preferred foliation of space-time, which breaks Lorentz
invariance. In contrast to the relativistic case, covariant Lifshitz theories
are only invariant under diffeomorphisms preserving the foliation structure. We
develop a systematic method to reduce the calculation of the effective action
for a generic Lifshitz operator to an algorithm acting on known results for
relativistic operators. In addition, we present techniques that drastically
simplify the calculation for operators with special properties. We demonstrate
the efficiency of these methods by explicit applications.Comment: 36 pages, matches journal versio
The Index Distribution of Gaussian Random Matrices
We compute analytically, for large N, the probability distribution of the
number of positive eigenvalues (the index N_{+}) of a random NxN matrix
belonging to Gaussian orthogonal (\beta=1), unitary (\beta=2) or symplectic
(\beta=4) ensembles. The distribution of the fraction of positive eigenvalues
c=N_{+}/N scales, for large N, as Prob(c,N)\simeq\exp[-\beta N^2 \Phi(c)] where
the rate function \Phi(c), symmetric around c=1/2 and universal (independent of
), is calculated exactly. The distribution has non-Gaussian tails, but
even near its peak at c=1/2 it is not strictly Gaussian due to an unusual
logarithmic singularity in the rate function.Comment: 4 pages Revtex, 4 .eps figures include
Topological Measure Locating the Effective Crossover between Segregation and Integration in a Modular Network
We introduce an easily computable topological measure which locates the
effective crossover between segregation and integration in a modular network.
Segregation corresponds to the degree of network modularity, while integration
is expressed in terms of the algebraic connectivity of an associated
hyper-graph. The rigorous treatment of the simplified case of cliques of equal
size that are gradually rewired until they become completely merged, allows us
to show that this topological crossover can be made to coincide with a
dynamical crossover from cluster to global synchronization of a system of
coupled phase oscillators. The dynamical crossover is signaled by a peak in the
product of the measures of intra-cluster and global synchronization, which we
propose as a dynamical measure of complexity. This quantity is much easier to
compute than the entropy (of the average frequencies of the oscillators), and
displays a behavior which closely mimics that of the dynamical complexity index
based on the latter. The proposed toplogical measure simultaneously provides
information on the dynamical behavior, sheds light on the interplay between
modularity vs total integration and shows how this affects the capability of
the network to perform both local and distributed dynamical tasks
Estimation procedures affect the center of pressure frequency analysis
Even though frequency analysis of body sway is widely applied in clinical studies, the lack of standardized proceduresconcerning power spectrum estimation may provide unreliable descriptors. Stabilometric tests were applied to 35 subjects (20-51 years, 54-95 kg, 1.6-1.9 m) and the power spectral density function was estimated for the anterior-posterior center of press uretime series. The median frequency was compared between power spectra estimated according to signal partitioning, samplingrate, test duration, and detrending methods. The median frequency reliability for different test durations was assessed using t heintraclass correlation coefficient. When increasing number of segments, shortening test duration or applying linear detrending,the median frequency values increased significantly up to 137%. Even the shortest test duration provided reliable estimates asobserved with the intraclass coefficient (0.74-0.89 confidence interval for a single 20-s test). Clinical assessment of balance maybenefit from a standardized protocol for center of pressure spectral analysis that provides an adequate relationship betweenresolution and variance. An algorithm to estimate center of pressure power density spectrum is also proposed.Key words: Center of pressure; Spectral analysis; Quiet standing; Estimators; Median frequencyResearch partially supported by CNPq, Fundacao Universitaria Jose Bonifacio (FUJB), and FAPERJ. T.M.M. Vieira was therecipient of MSc scholarship from FAPERJ and CNPq.Received July 18, 2008. Accepted March 3, 200
Intense myocyte formation from cardiac stem cells in human cardiac hypertrophy
It is generally believed that increase in adult contractile cardiac mass can be accomplished only by hypertrophy of existing myocytes. Documentation of myocardial regeneration in acute stress has challenged this dogma and led to the proposition that myocyte renewal is fundamental to cardiac homeostasis. Here we report that in human aortic stenosis, increased cardiac mass results from a combination of myocyte hypertrophy and hyperplasia. Intense new myocyte formation results from the differentiation of stem-like cells committed to the myocyte lineage. These cells express stem cell markers and telomerase. Their number increased >13-fold in aortic stenosis. The finding of cell clusters with stem cells making the transition to cardiogenic and myocyte precursors, as well as very primitive myocytes that turn into terminally differentiated myocytes, provides a link between cardiac stem cells and myocyte differentiation. Growth and differentiation of these primitive cells was markedly enhanced in hypertrophy, consistent with activation of a restricted number of stem cells that, through symmetrical cell division, generate asynchronously differentiating progeny. These clusters strongly support the existence of cardiac stem cells that amplify and commit to the myocyte lineage in response to increased workload. Their presence is consistent with the notion that myocyte hyperplasia significantly contributes to cardiac hypertrophy and accounts for the subpopulation of cycling myocytes
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