1,374 research outputs found
Channel Density Regulation of Firing Patterns in a Cortical Neuron Model
AbstractModifying the density and distribution of ion channels in a neuron (by natural up- and downregulation or by pharmacological intervention or by spontaneous mutations) changes its activity pattern. In this investigation we analyzed how the impulse patterns are regulated by the density of voltage-gated channels in a neuron model based on voltage-clamp measurements of hippocampal interneurons. At least three distinct oscillatory patterns, associated with three distinct regions in the Na-K channel density plane, were found. A stability analysis showed that the different regions are characterized by saddle-node, double-orbit, and Hopf-bifurcation threshold dynamics, respectively. Single, strongly graded action potentials occur in an area outside the oscillatory regions, but less graded action potentials occur together with repetitive firing over a considerable range of channel densities. The relationship found here between channel densities and oscillatory behavior may partly explain the difference between the principal spiking patterns previously described for crab axons (class 1 and 2) and cortical neurons (regular firing and fast spiking)
Distorted Copulas: Constructions and Tail Dependence
Given a copula C, we examine under which conditions on an order isomorphism ψ of [0, 1] the distortion C ψ: [0, 1]2 → [0, 1], C ψ(x, y) = ψ{C[ψ−1(x), ψ−1(y)]} is again a copula. In particular, when the copula C is totally positive of order 2, we give a sufficient condition on ψ that ensures that any distortion of C by means of ψ is again a copula. The presented results allow us to introduce in a more flexible way families of copulas exhibiting different behavior in the tails
Preassociative aggregation functions
The classical property of associativity is very often considered in
aggregation function theory and fuzzy logic. In this paper we provide
axiomatizations of various classes of preassociative functions, where
preassociativity is a generalization of associativity recently introduced by
the authors. These axiomatizations are based on existing characterizations of
some noteworthy classes of associative operations, such as the class of
Acz\'elian semigroups and the class of t-norms.Comment: arXiv admin note: text overlap with arXiv:1309.730
Additive generators based on generalized arithmetic operators in interval-valued fuzzy and Atanassov's intuitionistic fuzzy set theory
In this paper we investigate additive generators in Atanassov's intuitionistic fuzzy and interval-valued fuzzy set theory. Starting from generalized arithmetic operators satisfying some axioms we define additive generators and we characterize continuous generators which map exact elements to exact elements in terms of generators on the unit interval. We give necessary and sufficient condition under which a generator actually generates a t-nporm and we show that the generated t-norm belongs to particular classes of t-norms depending on the arithmetic operators involved in the defintion of the generator
Revealing the structure of the outer disks of Be stars
Context. The structure of the inner parts of Be star disks (20 stellar radii)
is well explained by the viscous decretion disk (VDD) model, which is able to
reproduce the observable properties of most of the objects studied so far. The
outer parts, on the ther hand, are not observationally well-explored, as they
are observable only at radio wavelengths. A steepening of the spectral slope
somewhere between infrared and radio wavelengths was reported for several Be
stars that were previously detected in the radio, but a convincing physical
explanation for this trend has not yet been provided. Aims. We test the VDD
model predictions for the extended parts of a sample of six Be disks that have
been observed in the radio to address the question of whether the observed
turndown in the spectral energy distribution (SED) can be explained in the
framework of the VDD model, including recent theoretical development for
truncated Be disks in binary systems. Methods. We combine new multi-wavelength
radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama
Pathfinder Experiment (APEX) with previously published radio data and archival
SED measurements at ultraviolet, visual, and infrared wavelengths. The density
structure of the disks, including their outer parts, is constrained by
radiative transfer modeling of the observed spectrum using VDD model
predictions. In the VDD model we include the presumed effects of possible tidal
influence from faint binary companions. Results. For 5 out of 6 studied stars,
the observed SED shows strong signs of SED turndown between far-IR and radio
wavelengths. A VDD model that extends to large distances closely reproduces the
observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED.
... (abstract continues but did not fit here)Comment: 20 pages, 8 figure
Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star CMi
The viscous decretion disk (VDD) model is able to explain most of the
currently observable properties of the circumstellar disks of Be stars.
However, more stringent tests, focusing on reproducing multitechnique
observations of individual targets via physical modeling, are needed to study
the predictions of the VDD model under specific circumstances. In the case of
nearby, bright Be star CMi, these circumstances are a very stable
low-density disk and a late-type (B8Ve) central star. The aim is to test the
VDD model thoroughly, exploiting the full diagnostic potential of individual
types of observations, in particular, to constrain the poorly known structure
of the outer disk if possible, and to test truncation effects caused by a
possible binary companion using radio observations. We use the Monte Carlo
radiative transfer code HDUST to produce model observables, which we compare
with a very large set of multitechnique and multiwavelength observations that
include ultraviolet and optical spectra, photometry covering the interval
between optical and radio wavelengths, optical polarimetry, and optical and
near-IR (spectro)interferometry. Due to the absence of large scale variability,
data from different epochs can be combined into a single dataset. A parametric
VDD model with radial density exponent of = 3.5, which is the canonical
value for isothermal flaring disks, is found to explain observables typically
formed in the inner disk, while observables originating in the more extended
parts favor a shallower, = 3.0, density falloff. Modeling of radio
observations allowed for the first determination of the physical extent of a Be
disk (35 stellar radii), which might be caused by a binary
companion. Finally, polarization data allowed for an indirect measurement of
the rotation rate of the star, which was found to be , i.e.,
very close to critical.Comment: 19 pages (35 including online material), 17 figures, 2 online
figures, 2 online tables with dat
Effect of the process gas and scan speed on the properties and productivity of thin 316L structures produced by Laser-Powder Bed Fusion
The development of the laser powder bed fusion (L-PBF) process to increase its robustness and productivity is challenged by ambitious design optimizations, such as thin wall structures. In this study, in addition to the effect of commonly used gases as Ar and N2, increased laser scanning speed and new process gases, such as helium, were successfully implemented. This implementation allowed to build 316L stainless steel components with thin walls of 1 mm thickness with an enhanced build rate of 37 pct. The sample size effect and the surface roughness were held responsible for the reduction in strength (YS > 430 MPa) and elongation (EAB > 30 pct) for the 1 mm samples studied. Similar strength was achieved for all process gases. The increased scanning speed was accompanied by a more random texture, smaller cell size, and grain size factor along the building direction when compared to the material built with the standard laser parameters. Stronger preferential orientation 〈101〉 along the building direction was observed for material built with standard parameters. Finally, the use of helium as a process gas was successful and resulted in reduced cell size. This finding is promising for the future development of high strength 316L stainless steel built with high build rates
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