34,044 research outputs found
Are there laws of genome evolution?
Research in quantitative evolutionary genomics and systems biology led to the
discovery of several universal regularities connecting genomic and molecular
phenomic variables. These universals include the log-normal distribution of the
evolutionary rates of orthologous genes; the power law-like distributions of
paralogous family size and node degree in various biological networks; the
negative correlation between a gene's sequence evolution rate and expression
level; and differential scaling of functional classes of genes with genome
size. The universals of genome evolution can be accounted for by simple
mathematical models similar to those used in statistical physics, such as the
birth-death-innovation model. These models do not explicitly incorporate
selection, therefore the observed universal regularities do not appear to be
shaped by selection but rather are emergent properties of gene ensembles.
Although a complete physical theory of evolutionary biology is inconceivable,
the universals of genome evolution might qualify as 'laws of evolutionary
genomics' in the same sense 'law' is understood in modern physics.Comment: 17 pages, 2 figure
Gyrochronology and its usage for main sequence cool star ages
The construction of all age indicators consists of certain basic steps which
lead to the identification of the properties desirable for stellar age
indicators. Prior age indicators for main sequence field stars possess only
some of these properties. The measured rotation periods of cool stars are
particularly useful in this respect because they have well-defined dependencies
that allow stellar ages to be determined with ~20% errors. This method, called
gyrochronology, is explained informally in this talk, shown to have the desired
properties, compared to prior methods, and used to derive ages for samples of
main sequence field stars.Comment: 10 pages, 6 figures, presented at IAU258, Ages of Star
Influence of e-e scattering on the temperature dependence of the resistance of a classical ballistic point contact in a two-dimensional electron system
We experimentally investigate the temperature (T) dependence of the
resistance of a classical ballistic point contact (PC) in a two-dimensional
electron system (2DES). The split-gate PC is realized in a high-quality
AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than
10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the
T dependence is roughly linear below 2 K and tends to saturate at higher T.
Perpendicular magnetic fields on the order of a few 10 mT suppress the
T-dependent contribution dR. This effect is more pronounced at lower
temperatures, causing a crossover to a nearly parabolic T dependence in a
magnetic field. The normalized magnetic field dependencies dR(B) permit an
empiric single parameter scaling in a wide range of PC gate voltages. These
observations give strong evidence for the influence of electron-electron (e-e)
scattering on the resistance of ballistic PCs. Our results are in qualitative
agreement with a recent theory of the e-e scattering based T dependence of the
conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and
Phys. Rev. B 81 125316 (2010)].Comment: as publishe
Why is the snowflake schema a good data warehouse design?
Database design for data warehouses is based on the notion of the snowflake schema and its important special case, the star schema. The snowflake schema represents a dimensional model which is composed of a central fact table and a set of constituent dimension tables which can be further broken up into subdimension tables. We formalise the concept of a snowflake schema in terms of an acyclic database schema whose join tree satisfies certain structural properties. We then define a normal form for snowflake schemas which captures its intuitive meaning with respect to a set of functional and inclusion dependencies. We show that snowflake schemas in this normal form are independent as well as separable when the relation schemas are pairwise incomparable. This implies that relations in the data warehouse can be updated independently of each other as long as referential integrity is maintained. In addition, we show that a data warehouse in snowflake normal form can be queried by joining the relation over the fact table with the relations over its dimension and subdimension tables. We also examine an information-theoretic interpretation of the snowflake schema and show that the redundancy of the primary key of the fact table is zero
Random databases with correlated data
A model of random databases is given, with arbitrary correlations among the data of one individual. This is given by a joint distribution function. The individuals are chosen independently, their number m is considered to be (approximately) known. The probability of the event that a given functional dependency A → b holds (A is a set of attributes, b is an attribute) is determined in a limiting sense. This probability is small if m is much larger than and is large if m is much smaller than 2 H 2(A→b)/ 2 where H 2(A→b) is an entropy like functional of the probability distribution of the data. © 2012 Springer-Verlag Berlin Heidelberg
Monte Carlo Simulation of Magnetization Reversal in Fe Sesquilayers on W(110)
Iron sesquilayers grown at room temperature on W(110) exhibit a pronounced
coercivity maximum near a coverage of 1.5 atomic monolayers. On lattices which
faithfully reproduce the morphology of the real films, a kinetic Ising model is
utilized to simulate the domain-wall motion. Simulations reveal that the
dynamics is dominated by the second-layer islands, which act as pinning
centers. The simulated dependencies of the coercivity on the film coverage, as
well as on the temperature and the frequency of the applied field, are very
similar to those measured in experiments. Unlike previous micromagnetic models,
the presented approach provides insight into the dynamics of the domain-wall
motion and clearly reveals the role of thermal fluctuations.Comment: Final version to appear in Phys. Rev. B. References to related works
added. 7 pages, 5 figures, RevTex, mpeg simulations available at
http://www.scri.fsu.edu/~rikvol
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