3,041 research outputs found
Complexity and predictability of daily precipitation in a semi-arid region: an application to Ceará, Brazil
International audienceComplexity and predictability of daily precipitation in a tropical semi-arid region (Ceará State, Brazil) is assessed by applying entropy concepts. Precipitation regimes in that region depend on several dynamical forcings, the most important being the displacement and activity of the Inter-tropical Convergence Zone in the Atlantic Ocean. Topography is another important factor that influences the spatial distribution of rainfall in the region. A hierarchical approach based on sequences of events of different lengths is used to estimate complexity of daily precipitation records. It is shown that precipitation in Ceará exhibit more random than periodic sequences, which indicates a large degree of complexity. Nevertheless, there is indication of potentially inherent rules in the precipitation time-series that could ultimately improve prediction on time-scales between 9?11 days. It is suggested that synoptic-scale disturbances (1?8 days) represent important sources of rules in the precipitation regimes in this region
What is modified gravity and how to differentiate it from particle dark matter?
An obvious criterion to classify theories of modified gravity is to identify their gravitational degrees of freedom and their coupling to the metric and the matter sector. Using this simple idea, we show that any theory which depends on the curvature invariants is equivalent to general relativity in the presence of new fields that are gravitationally coupled to the energy-momentum tensor. We show that they can be shifted into a new energy-momentum tensor. There is no a priori reason to identify these new fields as gravitational degrees of freedom or matter fields. This leads to an equivalence between dark matter particles gravitationally coupled to the standard model fields and modified gravity theories designed to account for the dark matter phenomenon. Due to this ambiguity, it is impossible to differentiate experimentally between these theories and any attempt of doing so should be classified as a mere interpretation of the same phenomenon
Tensor gauge fields in arbitrary representations of GL(D,R): II. Quadratic actions
Quadratic, second-order, non-local actions for tensor gauge fields
transforming in arbitrary irreducible representations of the general linear
group in D-dimensional Minkowski space are explicitly written in a compact form
by making use of Levi-Civita tensors. The field equations derived from these
actions ensure the propagation of the correct massless physical degrees of
freedom and are shown to be equivalent to non-Lagrangian local field equations
proposed previously. Moreover, these actions allow a frame-like reformulation a
la MacDowell-Mansouri, without any trace constraint in the tangent indices.Comment: LaTeX, 53 pages, no figure. Accepted for publication in
Communications in Mathematical Physics. Local Fierz-Pauli programme achieved
by completing the analysis of Labastid
Semiclassical Approximations in Phase Space with Coherent States
We present a complete derivation of the semiclassical limit of the coherent
state propagator in one dimension, starting from path integrals in phase space.
We show that the arbitrariness in the path integral representation, which
follows from the overcompleteness of the coherent states, results in many
different semiclassical limits. We explicitly derive two possible semiclassical
formulae for the propagator, we suggest a third one, and we discuss their
relationships. We also derive an initial value representation for the
semiclassical propagator, based on an initial gaussian wavepacket. It turns out
to be related to, but different from, Heller's thawed gaussian approximation.
It is very different from the Herman--Kluk formula, which is not a correct
semiclassical limit. We point out errors in two derivations of the latter.
Finally we show how the semiclassical coherent state propagators lead to
WKB-type quantization rules and to approximations for the Husimi distributions
of stationary states.Comment: 80 pages, 4 figure
Ageing in bosonic particle-reaction models with long-range transport
Ageing in systems without detailed balance is studied in bosonic contact and
pair-contact processes with Levy diffusion. In the ageing regime, the dynamical
scaling of the two-time correlation function and two-time response function is
found and analysed. Exact results for non-equilibrium exponents and scaling
functions are derived. The behaviour of the fluctuation-dissipation ratio is
analysed. A passage time from the quasi-stationary regime to the ageing regime
is defined, in qualitative agreement with kinetic spherical models and p-spin
spherical glasses.Comment: Latex2e, 24 pages, with 9 figures include
Microevolution of Helicobacter pylori during prolonged infection of single hosts and within families
Our understanding of basic evolutionary processes in bacteria is still very limited. For example, multiple recent dating estimates are based on a universal inter-species molecular clock rate, but that rate was calibrated using estimates of geological dates that are no longer accepted. We therefore estimated the short-term rates of mutation and recombination in Helicobacter pylori by sequencing an average of 39,300 bp in 78 gene fragments from 97 isolates. These isolates included 34 pairs of sequential samples, which were sampled at intervals of 0.25 to 10.2 years. They also included single isolates from 29 individuals (average age: 45 years) from 10 families. The accumulation of sequence diversity increased with time of separation in a clock-like manner in the sequential isolates. We used Approximate Bayesian Computation to estimate the rates of mutation, recombination, mean length of recombination tracts, and average diversity in those tracts. The estimates indicate that the short-term mutation rate is 1.4×10−6 (serial isolates) to 4.5×10−6 (family isolates) per nucleotide per year and that three times as many substitutions are introduced by recombination as by mutation. The long-term mutation rate over millennia is 5–17-fold lower, partly due to the removal of non-synonymous mutations due to purifying selection. Comparisons with the recent literature show that short-term mutation rates vary dramatically in different bacterial species and can span a range of several orders of magnitude
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