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Constructing highly arc transitive digraphs using a layerwise direct product
Christoph Neumann
Mathematics , 2011,
Abstract: We introduce a construction of highly arc transitive digraphs using a layerwise direct product. This product generalizes some known classes of highly arc transitive digraphs but also allows to construct new such. We use the product to obtain counterexamples to a conjecture by Cameron, Praeger and Wormald on the structure of certain highly arc transitive digraphs.
A complexity theorem for the Novelli-Pak-Stoyanovskii algorithm
Christoph Neumann,Robin Sulzgruber
Mathematics , 2013,
Abstract: We describe two aspects of the behaviour of entries of tabloids during the application of the Novelli-Pak-Stoyanovskii algorithm. We derive two theorems which both contain a generalized version of a conjecture by Krattenthaler and M\"uller concerning the complexity of the Novelli-Pak-Stoyanovskii algorithm as corollary.
Playing jeu de taquin on d-complete posets
Lukas Riegler,Christoph Neumann
Mathematics , 2014,
Abstract: Using a modified version of jeu de taquin, Novelli, Pak and Stoyanovskii gave a bijective proof of the hook-length formula for counting standard Young tableaux of fixed shape. In this paper we consider a natural extension of jeu de taquin to arbitrary posets. Given a poset P, jeu de taquin defines a map from the set of bijective labelings of the poset elements with $\{1,2,...,|P|\}$ to the set of linear extensions of the poset. One question of particular interest is for which posets this map yields each linear extension equally often. We analyze the double-tailed diamond poset $D_{m,n}$ and show that uniform distribution is obtained if and only if $D_{m,n}$ is d-complete. Furthermore, we observe that the extended hook-length formula for counting linear extensions on d-complete posets provides a combinatorial answer to a seemingly unrelated question, namely: Given a uniformly random standard Young tableau of fixed shape, what is the expected value of the left-most entry in the second row?
Highly sensitive feature detection for high resolution LC/MS
Ralf Tautenhahn, Christoph B?ttcher, Steffen Neumann
BMC Bioinformatics , 2008, DOI: 10.1186/1471-2105-9-504
Abstract: We developed a new feature detection algorithm centWave for high-resolution LC/MS data sets, which collects regions of interest (partial mass traces) in the raw-data, and applies continuous wavelet transformation and optionally Gauss-fitting in the chromatographic domain. We evaluated our feature detection algorithm on dilution series and mixtures of seed and leaf extracts, and estimated recall, precision and F-score of seed and leaf specific features in two experiments of different complexity.The new feature detection algorithm meets the requirements of current metabolomics experiments. centWave can detect close-by and partially overlapping features and has the highest overall recall and precision values compared to the other algorithms, matchedFilter (the original algorithm of XCMS) and the centroidPicker from MZmine. The centWave algorithm was integrated into the Bioconductor R-package XCMS and is available from http://www.bioconductor.org/ webciteMetabolomics aims at the unbiased and comprehensive quantification of metabolite concentrations in organisms, tissues, or cells [1,2]. The combination of chromatographic separation with subsequent mass spectrometric detection has emerged as a key technology for multiparallel analysis of low molecular weight compounds in biological systems. Gas chromatography-mass spectrometry (GC/MS) based techniques are mature and well-established, but restricted to volatile compounds, often requiring chemical derivatisation. High-performance liquid chromatography-mass spectrometry (HPLC/MS) facilitates the analysis of compounds of higher polarity and lower volatility in a much wider mass range without derivatisation [3-5]. With LC/MS the injected sample is separated on the chromatographic column, resulting in the consecutive elution of different compounds. The mass spectrometer acquires mass spectra from the column output at a specified scan rate, so each compound can be measured in several consecutive scans. Due to the fact that each
Wie k nnen Wikis im E-Learning ihr Potential entfalten?
Christoph Koenig,Antje Müller,Julia Neumann
Kommunikation@gesellschaft , 2007,
Abstract: In diesem Beitrag wird ein Rückkopplungsmodell entwickelt, das die Dynamik von Wikis in ‘freier Wildbahn’ am Beispiel von Open Source Projekten beschreibt. Es werden Erfahrungen aus einem Feldversuch geschildert, in dem diese Dynamik im Kontext von universit rem ELearning angeregt wurde. Aus diesen Erfahrungen werden drei typische Dynamiken für Wikis im E-Learning abgeleitet, aus denen Konsequenzen gezogen werden, wie die Arbeit im Wiki und die Lehre sich ver ndern, wenn sich im E-Learning die selben Potentiale wie in der ‘freie Wildbahn’ entfalten sollen.
Tackling CASMI 2012: Solutions from MetFrag and MetFusion
Christoph Ruttkies,Michael Gerlich,Steffen Neumann
Metabolites , 2013, DOI: 10.3390/metabo3030623
Abstract: The task in the critical assessment of small molecule identification (CASMI) contest category 2 was to determine the identification of (initially) unknown compounds for which high-resolution tandem mass spectra were published. We focused on computer-assisted methods that tried to correctly identify the compound automatically and entered the contest with MetFrag and MetFusion to score candidate structures retrieved from the PubChem structure database. MetFrag was combined with the metabolite-likeness score, which helped to improve the performance for the natural product challenges. We present the results, discuss the performance, and give details of how to interpret the MetFrag and MetFusion output.
DNStamp: Short-lived Trusted Timestamping
Christoph Neumann,Olivier Heen,Stéphane Onno
Computer Science , 2013,
Abstract: Trusted timestamping consists in proving that certain data existed at a particular point in time. Existing timestamping methods require either a centralized and dedicated trusted service or the collaboration of other participants using the timestamping service. We propose a novel trusted timestamping scheme, called DNStamp, that does not require a dedicated service nor collaboration between participants. DNStamp produces shortlived timestamps with a validity period of several days. The generation and verification involves a large number of Domain Name System cache resolvers, thus removing any single point of failure and any single point of trust. Any host with Internet access may request or verify a timestamp, with no need to register to any timestamping service. We provide a full description and analysis of DNStamp. We analyze the security against various adversaries and show resistance to forward-dating, back-dating and erasure attacks. Experiments with our implementation of DNStamp show that one can set and then reliably verify timestamps even under continuous attack conditions.
Cache policies for cloud-based systems: To keep or not to keep
Nicolas Le Scouarnec,Christoph Neumann,Gilles Straub
Computer Science , 2013,
Abstract: In this paper, we study cache policies for cloud-based caching. Cloud-based caching uses cloud storage services such as Amazon S3 as a cache for data items that would have been recomputed otherwise. Cloud-based caching departs from classical caching: cloud resources are potentially infinite and only paid when used, while classical caching relies on a fixed storage capacity and its main monetary cost comes from the initial investment. To deal with this new context, we design and evaluate a new caching policy that minimizes the overall cost of a cloud-based system. The policy takes into account the frequency of consumption of an item and the cloud cost model. We show that this policy is easier to operate, that it scales with the demand and that it outperforms classical policies managing a fixed capacity.
An empirical study of passive 802.11 Device Fingerprinting
Christoph Neumann,Olivier Heen,Stéphane Onno
Computer Science , 2014,
Abstract: 802.11 device fingerprinting is the action of characterizing a target device through its wireless traffic. This results in a signature that may be used for identification, network monitoring or intrusion detection. The fingerprinting method can be active by sending traffic to the target device, or passive by just observing the traffic sent by the target device. Many passive fingerprinting methods rely on the observation of one particular network feature, such as the rate switching behavior or the transmission pattern of probe requests. In this work, we evaluate a set of global wireless network parameters with respect to their ability to identify 802.11 devices. We restrict ourselves to parameters that can be observed passively using a standard wireless card. We evaluate these parameters for two different tests: i) the identification test that returns one single result being the closest match for the target device, and ii) the similarity test that returns a set of devices that are close to the target devices. We find that the network parameters transmission time and frame inter-arrival time perform best in comparison to the other network parameters considered. Finally, we focus on inter-arrival times, the most promising parameter for device identification, and show its dependency from several device characteristics such as the wireless card and driver but also running applications.
Local transport measurements on epitaxial graphene
Jens Baringhaus,Frederik Edler,Christoph Neumann,Christoph Stampfer,Stiven Forti,Ulrich Starke,Christoph Tegenkamp
Physics , 2013, DOI: 10.1063/1.4821364
Abstract: Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip STM/SEM the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.
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