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Search Results: 1 - 10 of 1408 matches for " Thorsten Stahn "
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Existence of solutions for semilinear elliptic boundary value problems on arbitrary open sets
Reinhard Stahn
Mathematics , 2015,
Abstract: We show the existence of a weak solution of a semilinear elliptic Dirichlet problem on an arbitrary open set. We make no assumptions about the open set, very mild regularity assumptions on the semilinearity, plus a coerciveness assumption which depends on the optimal Poincare-Steklov constant. The proof is based on Schaefer's fixed point theorem applied to a sequence of truncated problems. We state a simple uniqueness result. We also generalize the results to Robin boundary conditions.
Sequential Grouping Modulates the Effect of Non-Simultaneous Masking on Auditory Intensity Resolution
Daniel Oberfeld, Patricia Stahn
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0048054
Abstract: The presence of non-simultaneous maskers can result in strong impairment in auditory intensity resolution relative to a condition without maskers, and causes a complex pattern of effects that is difficult to explain on the basis of peripheral processing. We suggest that the failure of selective attention to the target tones is a useful framework for understanding these effects. Two experiments tested the hypothesis that the sequential grouping of the targets and the maskers into separate auditory objects facilitates selective attention and therefore reduces the masker-induced impairment in intensity resolution. In Experiment 1, a condition favoring the processing of the maskers and the targets as two separate auditory objects due to grouping by temporal proximity was contrasted with the usual forward masking setting where the masker and the target presented within each observation interval of the two-interval task can be expected to be grouped together. As expected, the former condition resulted in a significantly smaller masker-induced elevation of the intensity difference limens (DLs). In Experiment 2, embedding the targets in an isochronous sequence of maskers led to a significantly smaller DL-elevation than control conditions not favoring the perception of the maskers as a separate auditory stream. The observed effects of grouping are compatible with the assumption that a precise representation of target intensity is available at the decision stage, but that this information is used only in a suboptimal fashion due to limitations of selective attention. The data can be explained within a framework of object-based attention. The results impose constraints on physiological models of intensity discrimination. We discuss candidate structures for physiological correlates of the psychophysical data.
Implant-related MRI artifacts of determined interbody test spacers: artifact calculations due to implant parameters in a porcine spine model  [PDF]
Thorsten Ernstberger
Health (Health) , 2009, DOI: 10.4236/health.2009.13035
Abstract: Aim: Intervertebral spacers for anterior spine fusion are made of different materials, which can affect the post-fusion MRI scans. Suscep- tibility artifacts specially for implants made of titanium alloys can decrease the image quality. This study focused on the influence of deter-mined implant parameters like shape and implant volume in MRI artifacting independent from se-lected MRI-sequences. Methods: In this study the post-implantation MRI scans of determined cuboids and cylinders were evaluated. All in-terbody test implants were made of titanium alloys. MRI scans were carried out by using T1 TSE sequences. The total artifact volume (TAV) of all examined implants were calculated for sta-tistical t-test correlation and implant volume (IV)/TAV-relation. Results: Considering all ex-amined test implants with an increasing implant size the TAV became significant larger (p<0,001) with simultaneous reduction of the respective IV/TAV-relation. According to an intergroup TAV- correlation for cylinders and cuboids with an equivalent implant volume the cylindric test im-plants demonstrated a significant smaller arti-fact range (p<0,05). Conclusions: Based on these results the MRI artifacts of larger test im-plants were more limited to the to the implant’s direct surroundings. In this connection for im- plants with identical material volumes a cylin- dric shape demonstrated more advantages con- sidering MRI artifacting than cubic forms.
A Geometric Perspective on First-Passage Competition
Nathaniel D. Blair-Stahn
Mathematics , 2012,
Abstract: We study the macroscopic geometry of first-passage competition on the integer lattice $Z^d$, with a particular interest in describing the behavior when one species initially occupies the exterior of a cone. First-passage competition is a stochastic process modeling two infections spreading outward from initially occupied disjoint subsets of $Z^d$. Each infecting species transmits its infection at random times from previously infected sites to neighboring uninfected sites. The infection times are governed by species-specific probability distributions, and every vertex of $Z^d$ remains permanently infected by whichever species infects it first. We introduce a new, simple construction of first-passage competition that works for an arbitrary pair of disjoint starting sets in $Z^d$, and we analogously define a deterministic first-passage competition process in the Euclidean space $R^d$, providing a formal definition for a model of crystal growth that has previously been studied computationally. We then prove large deviations estimates for the random $Z^d$-process, showing that on large scales it is well-approximated by the deterministic $R^d$-process, with high probability. Analyzing the geometry of the deterministic process allows us to identify critical phenomena in the random process when one of the two species initially occupies the entire exterior of a cone and the other species initially occupies a single interior site. Our results generalize those in a 2007 paper of Deijfen and H\"aggstr\"om, who considered the case where the cone is a half-space. Moreover, we use our results about competition in cones to strengthen a 2000 result of H\"aggstr\"om and Pemantle about competition from finite starting configurations.
First passage percolation and competition models
Nathaniel D. Blair-Stahn
Mathematics , 2010,
Abstract: This paper is a survey of various results and techniques in first passage percolation, a random process modeling a spreading fluid on an infinite graph. The latter half of the paper focuses on the connection between first passage percolation and a certain class of stochastic growth and competition models.
Beacon-driven Leader Based Protocol over a GE Channel for MAC Layer Multicast Error Control  [PDF]
Zhao LI, Thorsten HERFET
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2008, DOI: 10.4236/ijcns.2008.12019
Abstract: In wireless networks current standard MAC layer protocols don’t provide any error correction scheme for broadcast/multicast. In this paper, we enhance a Leader Based Protocol (LBP) and propose a Beacon-driven Leader Based Protocol (BLBP) for the MAC layer multicast error control. To guarantee a very low Packet Loss Ratio (PLR) under strict delay constraints for video multicast over a Gilbert-Elliott (GE) channel, we analyze BLBP and compare it with LBP and different application layer multicast error control schemes via simulation experiments. Both the theoretical analysis and simulation results show that BLBP can correct nearly all the errors for all receivers in the MAC layer and is more efficient than LBP. BLBP is also more efficient than the application layer Automatic Repeat request (ARQ) scheme and the total multicast delay is much shorter. BLBP is very good for real-time multicast applications with strict delay constraints.
A Novel Adaptive Hybrid Error Correction Scheme for Wireless DVB Services  [PDF]
Guoping TAN, Thorsten HERFET
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2008, DOI: 10.4236/ijcns.2008.12023
Abstract: Real-time applications usually not only have a certain Packet Loss Ratio (PLR) requirement but also can have strict delay constraints. In the past, we proposed a Hybrid Error Correction (HEC) scheme with Packet Repetition (PR) technique for guaranteeing a certain PLR requirement under strict delay constraints. Unfortunately, the HEC-PR scheme can only work efficiently in multicast scenarios with small group size and small link PLR. Our further studies show that better performance can be obtained by combining the HEC-PR scheme with other traditional HEC schemes such as Type I HARQ and Type II HARQ techniques. Based on this idea, in this paper, a novel Adaptive HEC (AHEC) scheme combining the HEC-PR scheme with Type I and Type II HARQ techniques is proposed to satisfy a certain PLR requirement for delay bounded multicast services. Furthermore, the performance of the AHEC scheme is optimized by choosing the scheme with the least needed redundancy information automatically among the three HEC schemes. Finally, by applying the AHEC scheme in a typical wireless DVB scenario, we analyze the performances of the AHEC scheme and compare it with the HEC-PR scheme and an Adaptive Forward Error Correction (AFEC) scheme. The results show that the proposed AHEC scheme outperforms both the AFEC scheme and the HEC-PR scheme.
EXOTIME: searching for planets around pulsating subdwarf B stars
Sonja Schuh,Roberto Silvotti,Ronny Lutz,Bjoern Loeptien,Elizabeth M. Green,Roy H. Ostensen,Silvio Leccia,Seung-Lee Kim,Gilles Fontaine,Stephane Charpinet,Myriam Francoeur,Suzanna Randall,Cristina Rodriguez-Lopez,Valerie van Grootel,Andrew P. Odell,Margit Paparo,Zsofia Bognar,Peter Papics,Thorsten Nagel,Benjamin Beeck,Markus Hundertmark,Thorsten Stahn,Stefan Dreizler,Frederic V. Hessman,Massimo Dall'Ora,Dario Mancini,Fausto Cortecchia,Serena Benatti,Riccardo Claudi,Rimvydas Janulis
Physics , 2010, DOI: 10.1007/s10509-010-0356-4
Abstract: In 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undiscovered population of substellar companions to apparently single subdwarf B stars might exist. Following this serendipitous discovery, the EXOTIME (http://www.na.astro.it/~silvotti/exotime/) monitoring program has been set up to follow the pulsations of a number of selected rapidly pulsating subdwarf B stars on time-scales of several years with two immediate observational goals: 1) determine Pdot of the pulsational periods P 2) search for signatures of substellar companions in O-C residuals due to periodic light travel time variations, which would be tracking the central star's companion-induced wobble around the center of mass. These sets of data should therefore at the same time: on the one hand be useful to provide extra constraints for classical asteroseismological exercises from the Pdot (comparison with "local" evolutionary models), and on the other hand allow to investigate the preceding evolution of a target in terms of possible "binary" evolution by extending the otherwise unsuccessful search for companions to potentially very low masses. While timing pulsations may be an observationally expensive method to search for companions, it samples a different range of orbital parameters, inaccessible through orbital photometric effects or the radial velocity method: the latter favours massive close-in companions, whereas the timing method becomes increasingly more sensitive towards wider separations. In this paper we report on the status of the on-going observations and coherence analysis for two of the currently five targets, revealing very well-behaved pulsational characteristics in HS 0444+0458, while showing HS 0702+6043 to be more complex than previously thought.
Why Do Forward Maskers Affect Auditory Intensity Discrimination? Evidence from "Molecular Psychophysics"
Daniel Oberfeld, Patricia Stahn, Martha Kuta
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0099745
Abstract: Nonsimultaneous maskers can strongly impair performance in an auditory intensity discrimination task. Using methods of molecular psychophysics, we quantified the extent to which (1) a masker-induced impairment of the representation of target intensity (i.e., increase in internal noise) and (2) a systematic influence of the masker intensities on the decision variable contribute to these effects. In a two-interval intensity discrimination procedure, targets were presented in quiet, and combined with forward maskers. The lateralization of the maskers relative to the targets was varied via the interaural time difference. Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers. For most listeners and conditions, perceptual weights measuring the relation between the target and masker levels and the response in the intensity discrimination task were positive and significant. Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL. The maskers caused only a weak increase in internal noise, unrelated to target level and masker lateralization. The results indicate that the effects of forward masking on intensity discrimination are determined by an inclusion of the masker intensities in the decision variable, compatible with the hypothesis that the impairment in performance is to a large part caused by difficulties in directing selective attention to the targets. The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.
High intensity specular reflectometry - first experiments
J. Stahn,U. Filges,T. Panzner
Physics , 2011,
Abstract: Selene is the attempt to implement a new scheme for high-intensity specular reflectometry. Instead of a highly collimated beam one uses a convergent beam covering a large angular range. The angular resolution is then performed by a position-sensitive detector. Off-specular scattering in this set-up leads to some background, but for screening of wide parameter ranges (e.g. temperature, electric and magnetic fields) the intensity gain of at least one order of magnitude is essential. If necessary, the high precession measurements (even with off-specular components) then are performed with the conventional set-up. The heart of this new set-up is an elliptically focusing guide element of 2\,m length. Though this guide is optimised for the use on the TOF reflectometer Amor at SINQ, it can be used as stand-alone device to check the possible application also for other neutron scattering techniques. The first measurements on AMOR confirmed the general concept and the various operation modes. A draw-back occurred due to problems with the internal alignment of the guide. Nevertheless in the TOF mode a gain factor of 10 was reached, and a factor 25 can be expected for an improved guide.
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