21,779 research outputs found
Geometrical frustration yields fiber formation in self-assembly
Controlling the self-assembly of supramolecular structures is vital for
living cells, and a central challenge for engineering at the nano- and
microscales. Nevertheless, even particles without optimized shapes can robustly
form well-defined morphologies. This is the case in numerous medical conditions
where normally soluble proteins aggregate into fibers. Beyond the diversity of
molecular mechanisms involved, we propose that fibers generically arise from
the aggregation of irregular particles with short-range interactions. Using a
minimal model of ill-fitting, sticky particles, we demonstrate robust fiber
formation for a variety of particle shapes and aggregation conditions.
Geometrical frustration plays a crucial role in this process, and accounts for
the range of parameters in which fibers form as well as for their metastable
character.Comment: 6 pages, 5 figures, 1 ancillary movie; to appear in Nature Physic
On the area of constrained polygonal linkages
We study configuration spaces of linkages whose underlying graph are polygons
with diagonal constrains, or more general, partial two-trees. We show that
(with an appropriate definition) the oriented area is a Bott-Morse function on
the configuration space. Its critical points are described and Bott-Morse
indices are computed. This paper is a generalization of analogous results for
polygonal linkages (obtained earlier by G. Khimshiashvili, G. Panina, and A.
Zhukova)
Polygon scheduling
Consider a set of circles of the same length and r irregular polygons with vertices on a circle of this length. Each of the polygons has to be arranged on a given subset of all circles and the positions of the polygon on the different circles are depending on each other. How should the polygons be arranged relative to each other to minimize some criterion function depending on the distances between adjacent vertices on all circles? A decomposition of the set of all arrangements of the polygons into local regions in which the optimization problem is convex is given. An exact description of the local regions and a sharp bound on the number of local regions are derived. For the criterion functions minimizing the maximum weighted distance, maximizing the minimum weighted distance, and minimizing the sum of weighted distances the local optimization problems can be reduced to polynomially solvable network flow problems
Multi-Step Processing of Spatial Joins
Spatial joins are one of the most important operations for combining spatial objects of several relations. In this paper, spatial join processing is studied in detail for extended spatial objects in twodimensional data space. We present an approach for spatial join processing that is based on three steps. First, a spatial join is performed on the minimum bounding rectangles of the objects returning a set of candidates. Various approaches for accelerating this step of join processing have been examined at the last yearâs conference [BKS 93a]. In this paper, we focus on the problem how to compute the answers from the set of candidates which is handled by
the following two steps. First of all, sophisticated approximations
are used to identify answers as well as to filter out false hits from
the set of candidates. For this purpose, we investigate various types
of conservative and progressive approximations. In the last step, the
exact geometry of the remaining candidates has to be tested against
the join predicate. The time required for computing spatial join
predicates can essentially be reduced when objects are adequately
organized in main memory. In our approach, objects are first decomposed
into simple components which are exclusively organized
by a main-memory resident spatial data structure. Overall, we
present a complete approach of spatial join processing on complex
spatial objects. The performance of the individual steps of our approach
is evaluated with data sets from real cartographic applications.
The results show that our approach reduces the total execution
time of the spatial join by factors
Guidelines for assessing favourable conservation status of Natura 2000 species and habitat types in Bulgaria
This executive summary describes the methodology for assessing the favourable conservation status of N2000 habitats and species on site level in Bulgaria and gives guidelines for its application. The methodology was developed in the frame of the BBI/Matra project 2006/014 âFavourable Conservation Status of Natura 2000 Habitat types and Species in Bulgariaâ. The project was generously supported by the Dutch government under the BBI/Matra programme, which is a combination of two international policy programs of the Dutch government. The objectives and financial resources of the BBI/Matra Programme fall within the remit of the Matra Social Transformation Program of the Ministry of Foreign Affairs and under the International Policy Program on Biodiversity of the Ministry of Agriculture, Nature and Food Quality
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