6,786 research outputs found
Critical Line in Random Threshold Networks with Inhomogeneous Thresholds
We calculate analytically the critical connectivity of Random Threshold
Networks (RTN) for homogeneous and inhomogeneous thresholds, and confirm the
results by numerical simulations. We find a super-linear increase of with
the (average) absolute threshold , which approaches for large , and show that this asymptotic scaling is
universal for RTN with Poissonian distributed connectivity and threshold
distributions with a variance that grows slower than . Interestingly, we
find that inhomogeneous distribution of thresholds leads to increased
propagation of perturbations for sparsely connected networks, while for densely
connected networks damage is reduced; the cross-over point yields a novel,
characteristic connectivity , that has no counterpart in Boolean networks.
Last, local correlations between node thresholds and in-degree are introduced.
Here, numerical simulations show that even weak (anti-)correlations can lead to
a transition from ordered to chaotic dynamics, and vice versa. It is shown that
the naive mean-field assumption typical for the annealed approximation leads to
false predictions in this case, since correlations between thresholds and
out-degree that emerge as a side-effect strongly modify damage propagation
behavior.Comment: 18 figures, 17 pages revte
Self-organization of heterogeneous topology and symmetry breaking in networks with adaptive thresholds and rewiring
We study an evolutionary algorithm that locally adapts thresholds and wiring
in Random Threshold Networks, based on measurements of a dynamical order
parameter. A control parameter determines the probability of threshold
adaptations vs. link rewiring. For any , we find spontaneous symmetry
breaking into a new class of self-organized networks, characterized by a much
higher average connectivity than networks without threshold
adaptation (). While and evolved out-degree distributions
are independent from for , in-degree distributions become broader
when , approaching a power-law. In this limit, time scale separation
between threshold adaptions and rewiring also leads to strong correlations
between thresholds and in-degree. Finally, evidence is presented that networks
converge to self-organized criticality for large .Comment: 4 pages revtex, 6 figure
Damage Spreading and Criticality in Finite Random Dynamical Networks
We systematically study and compare damage spreading at the sparse
percolation (SP) limit for random boolean and threshold networks with
perturbations that are independent of the network size . This limit is
relevant to information and damage propagation in many technological and
natural networks. Using finite size scaling, we identify a new characteristic
connectivity , at which the average number of damaged nodes ,
after a large number of dynamical updates, is independent of . Based on
marginal damage spreading, we determine the critical connectivity
for finite at the SP limit and show that it
systematically deviates from , established by the annealed approximation,
even for large system sizes. Our findings can potentially explain the results
recently obtained for gene regulatory networks and have important implications
for the evolution of dynamical networks that solve specific computational or
functional tasks.Comment: 4 pages, 4 eps figure
Are geometric morphometric analyses replicable? Evaluating landmark measurement error and its impact on extant and fossil Microtus classification.
Geometric morphometric analyses are frequently employed to quantify biological shape and shape variation. Despite the popularity of this technique, quantification of measurement error in geometric morphometric datasets and its impact on statistical results is seldom assessed in the literature. Here, we evaluate error on 2D landmark coordinate configurations of the lower first molar of five North American Microtus (vole) species. We acquired data from the same specimens several times to quantify error from four data acquisition sources: specimen presentation, imaging devices, interobserver variation, and intraobserver variation. We then evaluated the impact of those errors on linear discriminant analysis-based classifications of the five species using recent specimens of known species affinity and fossil specimens of unknown species affinity. Results indicate that data acquisition error can be substantial, sometimes explaining >30% of the total variation among datasets. Comparisons of datasets digitized by different individuals exhibit the greatest discrepancies in landmark precision, and comparison of datasets photographed from different presentation angles yields the greatest discrepancies in species classification results. All error sources impact statistical classification to some extent. For example, no two landmark dataset replicates exhibit the same predicted group memberships of recent or fossil specimens. Our findings emphasize the need to mitigate error as much as possible during geometric morphometric data collection. Though the impact of measurement error on statistical fidelity is likely analysis-specific, we recommend that all geometric morphometric studies standardize specimen imaging equipment, specimen presentations (if analyses are 2D), and landmark digitizers to reduce error and subsequent analytical misinterpretations
Allometric trajectories of body and head morphology in three sympatric Arctic charr (Salvelinus alpinus (L.)) morphs
A study of body and head development in three sympatric reproductively isolated Arctic charr (Salvelinus alpinus (L.)) morphs from a subarctic lake (Skogsfjordvatn, northern Norway) revealed allometric trajectories that resulted in morphological differences. The three morphs were ecologically assigned to a littoral omnivore, a profundal benthivore and a profundal piscivore, and this was confirmed by genetic analyses (microsatellites). Principal component analysis was used to identify the variables responsible for most of the morphological variation of the body and head shape. The littoral omnivore and the profundal piscivore morph had convergent allometric trajectories for the most important head shape variables, developing bigger mouths and relatively smaller eyes with increasing head size. The two profundal morphs shared common trajectories for the variables explaining most of the body and head shape variation, namely head size relative to body size, placement of the dorsal and pelvic fins, eye size and mouth size. In contrast, the littoral omnivore and the profundal benthivore morphs were not on common allometric trajectories for any of the examined variables. The findings suggest that different selective pressures could have been working on traits related to their trophic niche such as habitat and diet utilization of the three morphs, with the two profundal morphs experiencing almost identical environmental conditions
Radiation Damage Studies of Silicon Photomultipliers
We report on the measurement of the radiation hardness of silicon
photomultipliers (SiPMs) manufactured by
Fondazione Bruno Kessler in Italy (1 mm and 6.2 mm), Center of
Perspective Technology and Apparatus in Russia (1 mm and 4.4 mm), and
Hamamatsu Corporation in Japan (1 mm). The SiPMs were irradiated using a
beam of 212 MeV protons at Massachusetts General Hospital, receiving fluences
of up to protons per cm with the SiPMs at operating
voltage. Leakage currents were read continuously during the irradiation. The
delivery of the protons was paused periodically to record scope traces in
response to calibrated light pulses to monitor the gains, photon detection
efficiencies, and dark counts of the SiPMs. The leakage current and dark noise
are found to increase with fluence. Te leakage current is found to be
proportional to the mean square deviation of the noise distribution, indicating
the dark counts are due to increased random individual pixel activation, while
SiPMs remain fully functional as photon detectors. The SiPMs are found to
anneal at room temperature with a reduction in the leakage current by a factor
of 2 in about 100 days.Comment: 35 pages, 25 figure
Multiple traits and multifarious environments:integrated divergence of morphology and life history
Observation of the Production of Jets of Particles at High Transverse Momentum and Comparison with Inclusive Single-Particle Reactions
Data are presented on production by 200-GeV/c hadrons incident on beryllium of both single particles and jets (groups of particles) with high p_T (transverse momentum). The experiment was performed in a wide-aperture multiparticle spectrometer at Fermilab. The jet and single-particle cross sections have a similar shape from p_T=3 to 5 GeV/c but the jet cross section is over two orders of magnitude larger. The distributions of charged-particle momenta show striking similarities to those observed in lepton-induced processes
Jets Produced in π^-, π^+, and Proton Interactions at 200 GeV on Hydrogen and Aluminum Targets
This paper presents results from an experiment on the production of jets (groups of particles) with high p_⊥ produced in 200-GeV/c interactions. Results are presented on the comparison of jet cross sections on aluminum and hydrogen targets. The jet fragmentation distributions are also examined. Both the cross section and the jet structure are found to depend strongly on the beam and target types
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