10,418 research outputs found
A qualitative approach to the identification, visualisation and interpretation of repetitive motion patterns in groups of moving point objects
Discovering repetitive patterns is important in a wide range of research areas, such as bioinformatics and human movement analysis. This study puts forward a new methodology to identify, visualise and interpret repetitive motion patterns in groups of Moving Point Objects (MPOs). The methodology consists of three steps. First, motion patterns are qualitatively described using the Qualitative Trajectory Calculus (QTC). Second, a similarity analysis is conducted to compare motion patterns and identify repetitive patterns. Third, repetitive motion patterns are represented and interpreted in a continuous triangular model. As an illustration of the usefulness of combining these hitherto separated methods, a specific movement case is examined: Samba dance, a rhythmical dance will? many repetitive movements. The results show that the presented methodology is able to successfully identify, visualize and interpret the contained repetitive motions
Finite frequency current noise in the Holstein model
We investigate the effects of local vibrational excitations in the
nonsymmetrized current noise of a nanojunction. For this purpose,
we analyze a simple model - the Holstein model - in which the junction is
described by a single electronic level that is coupled to two metallic leads
and to a single vibrational mode. Using the Keldysh Green's function technique,
we calculate the nonsymmetrized current noise to the leading order in the
charge-vibration interaction. For the noise associated to the latter, we
identify distinct terms corresponding to the mean-field noise and the vertex
correction. The mean-field result can be further divided into an elastic
correction to the noise and in an inelastic correction, the second one being
related to energy exchange with the vibration. To illustrate the general
behavior of the noise induced by the charge-vibration interaction, we consider
two limit cases. In the first case, we assume a strong coupling of the dot to
the leads with an energy-independent transmission whereas in the second case we
assume a weak tunneling coupling between the dot and the leads such that the
transport occurs through a sharp resonant level. We find that the noise
associated to the vibration-charge interaction shows a complex pattern as a
function of the frequency and of the transmission function or of the
dot's energy level. Several transitions from enhancement to suppression of the
noise occurs in different regions, which are determined, in particular, by the
vibrational frequency. Remarkably, in the regime of an energy-independent
transmission, the zero order elastic noise vanishes at perfect transmission and
at positive frequency whereas the noise related to the charge-vibration
interaction remains finite enabling the analysis of the pure
vibrational-induced current noise
Analysing imperfect temporal information in GIS using the Triangular Model
Rough set and fuzzy set are two frequently used approaches for modelling and reasoning about imperfect time intervals. In this paper, we focus on imperfect time intervals that can be modelled by rough sets and use an innovative graphic model [i.e. the triangular model (TM)] to represent this kind of imperfect time intervals. This work shows that TM is potentially advantageous in visualizing and querying imperfect time intervals, and its analytical power can be better exploited when it is implemented in a computer application with graphical user interfaces and interactive functions. Moreover, a probabilistic framework is proposed to handle the uncertainty issues in temporal queries. We use a case study to illustrate how the unique insights gained by TM can assist a geographical information system for exploratory spatio-temporal analysis
Diagrammatic expansion for positive spectral functions beyond GW: Application to vertex corrections in the electron gas
We present a diagrammatic approach to construct self-energy approximations
within many-body perturbation theory with positive spectral properties. The
method cures the problem of negative spectral functions which arises from a
straightforward inclusion of vertex diagrams beyond the GW approximation. Our
approach consists of a two-steps procedure: we first express the approximate
many-body self-energy as a product of half-diagrams and then identify the
minimal number of half-diagrams to add in order to form a perfect square. The
resulting self-energy is an unconventional sum of self-energy diagrams in which
the internal lines of half a diagram are time-ordered Green's functions whereas
those of the other half are anti-time-ordered Green's functions, and the lines
joining the two halves are either lesser or greater Green's functions. The
theory is developed using noninteracting Green's functions and subsequently
extended to self-consistent Green's functions. Issues related to the conserving
properties of diagrammatic approximations with positive spectral functions are
also addressed. As a major application of the formalism we derive the minimal
set of additional diagrams to make positive the spectral function of the GW
approximation with lowest-order vertex corrections and screened interactions.
The method is then applied to vertex corrections in the three-dimensional
homogeneous electron gas by using a combination of analytical frequency
integrations and numerical Monte-Carlo momentum integrations to evaluate the
diagrams.Comment: 19 pages, 19 figure
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