9,709 research outputs found
GLAND CELLS IN HYDRA
The proliferative capacity of gland cells in Hydra attenuata was investigated. The results
indicate that both gland cell proliferation and interstitial cell differentiation to gland cells
contribute to the maintenance of the whole population. On the basis of [3H]thymidine incorporation
and nuclear DNA measurements, gland cells consist of at least three different
populations. One population consists of rapidly proliferating cells with a cell cycle of about 72 h.
These cells are distributed throughout the body column. In the lower gastric region there is a
population of non-cycling cells in G2 while in the upper gastric region there is a population of noncycling
cells in G1. About half the G1 population becomes a new antigen, SEC 1, which is typical of
mucus cells
Low-momentum pion enhancement from schematic hadronization of a gluon-saturated initial state
We study the particle production in the early stage of the ultrarelativistic
heavy-ion collisions. To this end the Boltzmann kinetic equations for gluons
and pions with elastic rescattering are considered together with a simple model
for the parton-hadron conversion process (hadronisation). It is shown that the
overpopulation of the gluon phase space in the initial state leads to an
intermediate stage of Bose enhancement in the low-momentum gluon sector which
due to the gluon-pion conversion process is then reflected in the final
distribution function of pions. This pattern is very similar to the
experimental finding of a low-momentum pion enhancement in the ALICE experiment
at CERN LHC. Relations to the thermal statistical model of hadron production
and the phenomenon of thermal and chemical freeze-out are discussed in this
context
Interplay of spatial dynamics and local adaptation shapes species lifetime distributions and species-area relationships
The distributions of species lifetimes and species in space are related,
since species with good local survival chances have more time to colonize new
habitats and species inhabiting large areas have higher chances to survive
local disturbances. Yet, both distributions have been discussed in mostly
separate communities. Here, we study both patterns simultaneously using a
spatially explicit, evolutionary community assembly approach. We present and
investigate a metacommunity model, consisting of a grid of patches, where each
patch contains a local food web. Species survival depends on predation and
competition interactions, which in turn depend on species body masses as the
key traits. The system evolves due to the migration of species to neighboring
patches, the addition of new species as modifications of existing species, and
local extinction events. The structure of each local food web thus emerges in a
self-organized manner as the highly non-trivial outcome of the relative time
scales of these processes. Our model generates a large variety of complex,
multi-trophic networks and therefore serves as a powerful tool to investigate
ecosystems on long temporal and large spatial scales. We find that the observed
lifetime distributions and species-area relations resemble power laws over
appropriately chosen parameter ranges and thus agree qualitatively with
empirical findings. Moreover, we observe strong finite-size effects, and a
dependence of the relationships on the trophic level of the species. By
comparing our results to simple neutral models found in the literature, we
identify the features that are responsible for the values of the exponents.Comment: Theor Ecol (2019
Efficient Approximation of Quantum Channel Capacities
We propose an iterative method for approximating the capacity of
classical-quantum channels with a discrete input alphabet and a finite
dimensional output, possibly under additional constraints on the input
distribution. Based on duality of convex programming, we derive explicit upper
and lower bounds for the capacity. To provide an -close estimate
to the capacity, the presented algorithm requires , where denotes the input alphabet size and
the output dimension. We then generalize the method for the task of
approximating the capacity of classical-quantum channels with a bounded
continuous input alphabet and a finite dimensional output. For channels with a
finite dimensional quantum mechanical input and output, the idea of a universal
encoder allows us to approximate the Holevo capacity using the same method. In
particular, we show that the problem of approximating the Holevo capacity can
be reduced to a multidimensional integration problem. For families of quantum
channels fulfilling a certain assumption we show that the complexity to derive
an -close solution to the Holevo capacity is subexponential or
even polynomial in the problem size. We provide several examples to illustrate
the performance of the approximation scheme in practice.Comment: 36 pages, 1 figur
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