98 research outputs found
Random Convex Hulls and Extreme Value Statistics
In this paper we study the statistical properties of convex hulls of
random points in a plane chosen according to a given distribution. The points
may be chosen independently or they may be correlated. After a non-exhaustive
survey of the somewhat sporadic literature and diverse methods used in the
random convex hull problem, we present a unifying approach, based on the notion
of support function of a closed curve and the associated Cauchy's formulae,
that allows us to compute exactly the mean perimeter and the mean area enclosed
by the convex polygon both in case of independent as well as correlated points.
Our method demonstrates a beautiful link between the random convex hull problem
and the subject of extreme value statistics. As an example of correlated
points, we study here in detail the case when the points represent the vertices
of independent random walks. In the continuum time limit this reduces to
independent planar Brownian trajectories for which we compute exactly, for
all , the mean perimeter and the mean area of their global convex hull. Our
results have relevant applications in ecology in estimating the home range of a
herd of animals. Some of these results were announced recently in a short
communication [Phys. Rev. Lett. {\bf 103}, 140602 (2009)].Comment: 61 pages (pedagogical review); invited contribution to the special
issue of J. Stat. Phys. celebrating the 50 years of Yeshiba/Rutgers meeting
Learning from multimedia and hypermedia
Computer-based multimedia and hypermedia resources (e.g., the world wide web) have become one of the primary sources of academic information for a majority of pupils and students. In line with this expansion in the field of education, the scientific study of learning from multimedia and hypermedia has become a very active field of research. In this chapter we provide a short overview with regard to research on learning with multimedia and hypermedia. In two review sections, we describe the educational benefits of multiple representations and of learner control, as these are the two defining characteristics of hypermedia. In a third review section we describe recent scientific trends in the field of multimedia/hypermedia learning. In all three review sections we will point to relevant European work on multimedia/hypermedia carried out within the last 5 years, and often carried out within the Kaleidoscope Network of Excellence. According to the interdisciplinary nature of the field this work might come not only from psychology, but also from technology or pedagogy. Comparing the different research activities on multimedia and hypermedia that have dominated the international scientific discourse in the last decade reveals some important differences. Most important, a gap seems to exist between researchers mainly interested in a “serious” educational use of multimedia/ hypermedia and researchers mainly interested in “serious” experimental research on learning with multimedia/hypermedia. Recent discussions about the pros and cons of “design-based research” or “use-inspired basic research” can be seen as a direct consequence of an increasing awareness of the tensions within these two different cultures of research on education
The 2010 very high energy gamma-ray flare & 10 years of multi-wavelength observations of M 87
Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and
very massive black hole provides a unique opportunity to investigate the origin
of very high energy (VHE; E>100 GeV) gamma-ray emission generated in
relativistic outflows and the surroundings of super-massive black holes. M 87
has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray
emission displays strong variability on timescales as short as a day. In this
paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and
VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE
was detected triggering further observations at VHE (H.E.S.S.), X-rays
(Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray
light curve enables one to derive a precise temporal characterization of the
flare: the single, isolated flare is well described by a two-sided exponential
function with significantly different flux rise and decay times. While the
overall variability pattern of the 2010 flare appears somewhat different from
that of previous VHE flares in 2005 and 2008, they share very similar
timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2
s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions
indicate no enhanced flux in 2010 in contrast to observations in 2008, where an
increase of the radio flux of the innermost core regions coincided with a VHE
flare. On the other hand, Chandra X-ray observations taken ~3 days after the
peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The
long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio
to VHE and including data from HST, LT, VLA and EVN, is used to further
investigate the origin of the VHE gamma-ray emission. No unique, common MWL
signature of the three VHE flares has been identified.Comment: 19 pages, 5 figures; Corresponding authors: M. Raue, L. Stawarz, D.
Mazin, P. Colin, C. M. Hui, M. Beilicke; Fig. 1 lightcurve data available
online: http://www.desy.de/~mraue/m87
Ultramafic vegetation and soils in the circumboreal region of the Northern Hemisphere
The paper summarizes literature on climate, soil chemistry, vegetation and metal accumulation by plants found on ultramafic substrata in the circumboreal zone (sensu Takhtajan, Floristic regions of the world, 1986) of the Northern Hemisphere. We present a list of 50 endemic species and 18 ecotypes obligate to ultramafic soils from the circumboreal region of Holarctic, as well as 30 and 2 species of Ni and Zn hyperaccumulators, respectively. The number of both endemics and hyperaccumulators are markedly lower compared to that of the Mediterranean and tropical regions. The diversity of plant communities on ultramafics soils of the circumboral region is also described. The underlying causes for the differences of ultramafic flora between arctic, cold, cool temperate and Mediterranean and tropical regions are also discussed. © 2018, The Ecological Society of Japan
Textile Catalysts-An unconventional approach towards heterogeneous catalysis
Textile catalysts are a new approach utilizing immobilization of different classes of catalysts onto textile materials such as polyethylene terephthalate and polyamide. Robust, inexpensive fibrous materials are chosen because they are available in many variations. By a photochemical approach a series of different supported organocatalysts (organotextile catalysts) has been prepared, showing high catalytic activity and good reusability. The aim of this concept article is to present the scope, limits and open questions of our innovative approach. The working principle of the immobilization and its control parameters will be explained and the scope of useable catalysts is shown. Therefore we will show the significant influence of the anchoring group on loading and more importantly on catalyst activity. This concept is also applicable to organometallic catalysts and enzymes. Understanding the different phenomena allows us to develop “textile catalysts” as a new powerful tool for heterogeneous catalysis
Organotextile Catalysis
Throughout human history, textiles have been integral to daily life, but their exploration in catalysis has been rare. Herein, we show a facile and permanent immobilization of organocatalysts on the textile nylon using ultraviolet light. The catalyst and the textile material require no chemical modification for the immobilization. All of the prepared textile-immobilized organocatalysts (a Lewis basic, a Brønsted acidic, and a chiral organocatalyst) display excellent stability, activity, and recyclability for various organic transformations. Very good enantioselectivity (>95:5 enantiomeric ratio) can be maintained for more than 250 cycles of asymmetric catalysis. Practical and straightforward applications of textile organocatalysis may be beneficial for various fields by offering inexpensive and accessible functionalized catalytic materials
- …