29 research outputs found
Parallel implementation of the recursive Green’s function method,
Abstract A parallel algorithm for the implementation of the recursive Green's function technique, which is extensively applied in the coherent scattering formalism, is developed. The algorithm performs a domain decomposition of the scattering region among the processors participating in the computation and calculates the Schur's complement block in the form of distributed blocks among the processors. If the method is applied recursively, thereby eliminating the processors cyclically, it is possible to arrive at a Schur's complement block of small size and compute the desired block of the Green's function matrix directly. The numerical complexity due to the longitudinal dimension of the scatterer scales linearly with the number of processors, though, the computational cost due to the processors' cyclic reduction establishes a bottleneck to achieve efficiency 100%. The proposed algorithm is accompanied by a performance analysis for two numerical benchmarks, in which the dominant sources of computational load and parallel overhead as well as their competitive role in the efficiency of the algorithm will be demonstrated
Eigenvalue Problem in Two Dimensions for an Irregular Boundary II: Neumann Condition
We formulate a systematic elegant perturbative scheme for determining the
eigenvalues of the Helmholtz equation (\bigtriangledown^{2} + k^{2}){\psi} = 0
in two dimensions when the normal derivative of {\psi} vanishes on an irregular
closed curve. Unique feature of this method, unlike other perturbation schemes,
is that it does not require a separate formalism to treat degeneracies.
Degenerate states are handled equally elegantly as the non-degenerate ones. A
real parameter, extracted from the parameters defining the irregular boundary,
serves as a perturbation parameter in this scheme as opposed to earlier schemes
where the perturbation parameter is an artificial one. The efficacy of the
proposed scheme is gauged by calculating the eigenvalues for elliptical and
supercircular boundaries and comparing with the results obtained numerically.
We also present a simple and interesting semi-empirical formula, determining
the eigenspectrum of the 2D Helmholtz equation with the Dirichlet or the
Neumann condition for a supercircular boundary. A comparison of the
eigenspectrum for several low-lying modes obtained by employing the formula
with the corresponding numerical estimates shows good agreement for a wide
range of the supercircular exponent.Comment: 26 pages, 12 figure
Fast extraction of neuron morphologies from large-scale SBFSEM image stacks
Neuron morphology is frequently used to classify cell-types in the mammalian cortex. Apart from the shape of the soma and the axonal projections, morphological classification is largely defined by the dendrites of a neuron and their subcellular compartments, referred to as dendritic spines. The dimensions of a neuron’s dendritic compartment, including its spines, is also a major determinant of the passive and active electrical excitability of dendrites. Furthermore, the dimensions of dendritic branches and spines change during postnatal development and, possibly, following some types of neuronal activity patterns, changes depending on the activity of a neuron. Due to their small size, accurate quantitation of spine number and structure is difficult to achieve (Larkman, J Comp Neurol 306:332, 1991). Here we follow an analysis approach using high-resolution EM techniques. Serial block-face scanning electron microscopy (SBFSEM) enables automated imaging of large specimen volumes at high resolution. The large data sets generated by this technique make manual reconstruction of neuronal structure laborious. Here we present NeuroStruct, a reconstruction environment developed for fast and automated analysis of large SBFSEM data sets containing individual stained neurons using optimized algorithms for CPU and GPU hardware. NeuroStruct is based on 3D operators and integrates image information from image stacks of individual neurons filled with biocytin and stained with osmium tetroxide. The focus of the presented work is the reconstruction of dendritic branches with detailed representation of spines. NeuroStruct delivers both a 3D surface model of the reconstructed structures and a 1D geometrical model corresponding to the skeleton of the reconstructed structures. Both representations are a prerequisite for analysis of morphological characteristics and simulation signalling within a neuron that capture the influence of spines
Delegation and coordination with multiple threshold public goods: experimental evidence
When multiple charities, social programs and community projects simultaneously vie for funding, donors risk mis-coordinating their contributions leading to an inefficient distribution of funding across projects. Community chests and other intermediary organizations facilitate coordination among donors and reduce such risks. To study this, we extend a threshold public goods framework to allow donors to contribute through an intermediary rather than directly to the public goods. Through a series of experiments, we show that the presence of an intermediary increases public good success and subjects’ earnings only when the intermediary is formally committed to direct donations to socially beneficial goods. Without such a restriction, the presence of an intermediary has a negative impact, complicating the donation environment, decreasing contributions and public good success.When multiple charities, social programs and community projects simultaneously vie for funding, donors risk mis-coordinating their contributions leading to an inefficient distribution of funding across projects. Community chests and other intermediary organizations facilitate coordination among donors and reduce such risks. To study this, we extend a threshold public goods framework to allow donors to contribute through an intermediary rather than directly to the public goods. Through a series of experiments, we show that the presence of an intermediary increases public good success and subjects’ earnings only when the intermediary is formally committed to direct donations to socially beneficial goods. Without such a restriction, the presence of an intermediary has a negative impact, complicating the donation environment, decreasing contributions and public good success
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Motives and comprehension in a public goods game with induced emotions
This study analyses the sensitivity of public goods contributions through the lens of psychological motives. We report the results of a public goods experiment in which subjects were induced with the motives of care and anger through autobiographical recall. Subjects' preferences, beliefs, and perceptions under each motive are compared with those of subjects experiencing a neutral autobiographical recall control condition. We find, but only for those subjects with the highest comprehension of the game, that care elicits significantly higher contributions than anger, with the control treatment in between. This positive influence of the care motive on unconditional giving is accounted for partly by preferences for giving and partly by the beliefs concerning greater contributions by others. Anger also affects attention to own and other's payoffs (using mouse tracking) and perceptions of the game's incentive structure (cooperative or competitive)
Behavioral Economics and the Public Sector
This thesis consists of four essays dealing with topics that are relevant for the public sector. The essays cover diverse issues of economics partly overlapping with political science. The topics reach from the taxation of labor over monetary policy to preferences over voting institutions. Throughout this thesis it is, in contrast to classical economics, not assumed that humans are necessarily fully rational. Once full rationality is no longer assumed, experiments become an important tool to learn about human behavior. Consequently, most of the work in this thesis makes use of economic experiments
Two-electron anisotropic quantum dots
A detailed investigation of the effects of interaction and
anisotropy in the electronic structure and dynamical properties of
two-electron quantum dots is performed. It is shown that a small
anisotropy eliminates the shell structure and represents a rapid
path to chaos. The level clustering, energy gaps and the
accompanying classical dynamics are investigated among others for
the frequency ratios . For n=2 the
system is integrable and the corresponding constant of motion is
constructed. The eigenstates pair in singlet-triplet degenerate
subspaces. In between these ratios avoided crossings dominate the
spectra. For very strong anisotropies, the classical dot comprises
the complete regime from softly interacting to kicked
oscillators. Its quantum counterpart shows remarkable spectral
patterns