150 research outputs found
Genealogy of catalytic branching models
We consider catalytic branching populations. They consist of a catalyst
population evolving according to a critical binary branching process in
continuous time with a constant branching rate and a reactant population with a
branching rate proportional to the number of catalyst individuals alive. The
reactant forms a process in random medium. We describe asymptotically the
genealogy of catalytic branching populations coded as the induced forest of
-trees using the many individuals--rapid branching continuum limit.
The limiting continuum genealogical forests are then studied in detail from
both the quenched and annealed points of view. The result is obtained by
constructing a contour process and analyzing the appropriately rescaled version
and its limit. The genealogy of the limiting forest is described by a point
process. We compare geometric properties and statistics of the reactant limit
forest with those of the "classical" forest.Comment: Published in at http://dx.doi.org/10.1214/08-AAP574 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Manual measurement of retinal bifurcation features
This paper introduces a new computerized tool for
accurate manual measurement of features of retinal bifurcation
geometry, designed for use in investigating correlations between measurement features and clinical conditions. The tool uses user-placed rectangles to measure the vessel width, and lines placed along vessel center lines to measure the angles. An
analysis is presented of measurements taken from 435 bifurcations.
These are compared with theoretical predictions based on
optimality principles presented in the literature. The new tool shows better agreement with the theoretical predictions than a simpler manual method published in the literature, but there remains a significant discrepancy between current theory and measured geometry
A Hypergraph Model for Railway Vehicle Rotation Planning
We propose a model for the integrated optimization of vehicle rotations and vehicle compositions in long distance railway passenger transport. The main contribution of the paper is a hypergraph model that is able to handle the challenging technical requirements as well as very general stipulations with respect to the "regularity" of a schedule. The hypergraph model directly generalizes network flow models, replacing arcs with hyperarcs. Although NP-hard in general, the model is computationally well-behaved in practice. High quality solutions can be produced in reasonable time using high performance Integer Programming techniques, in particular, column generation and rapid branching. We show that, in this way, large-scale real world instances of our cooperation partner DB Fernverkehr can be solved
Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors
The optoelectronic properties of macromolecular semiconductors depend
fundamentally on their solid-state microstructure. For example, the
molecular-weight distribution influences polymeric- semiconductor properties
via diverse microstructures; polymers of low weight-average molecular weight
(Mw) form unconnected, extended-chain crystals, usually of a paraffinic
structure. Because of the non-entangled nature of the relatively short-chain
macromolecules, this leads to a polycrystalline, one-phase morphology. In
contrast, with high-Mw materials, where average chain lengths are longer than
the length between entanglements, two-phase morphologies, comprised of
crystalline moieties embedded in largely unordered (amorphous) regions, are
obtained. We investigate charge photogeneration processes in neat regioregular
poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved
photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination
of long-lived charge pairs decays over microsecond timescales. Both the
amplitude and decay rate distribution depend strongly on Mw. In films with
dominant one-phase chain-extended microstructures, the delayed PL is suppressed
as a result of a diminished yield of photoinduced charges, and its decay is
significantly faster than in two-phase microstructures. However, independent of
Mw, charge recombination regenerates singlet excitons in torsionally disordered
chains forming more strongly coupled photophysical aggregates than those in the
steady-state ensemble, with delayed PL lineshape reminiscent of that in
paraffinic morphologies at steady state. We conclude that highly delocalized
excitons in disordered regions between crystalline and amorphous phases
dissociate extrinsically with yield and spatial distribution that depend
intimately upon microstructure.Comment: 19 pages, 4 figure
Optimizing High-Speed Railroad Timetable with Passenger and Station Service Demands: A Case Study in the Wuhan-Guangzhou Corridor
This paper aims to optimize high-speed railroad timetables for a corridor. We propose an integer programming model using a time-space network-based approach to consider passenger service demands, train scheduling, and station service demands simultaneously. A modified branch-and-price algorithm is used for the computation. This algorithm solves the linear relaxation of all nodes in a branch-and-bound tree using a column generation algorithm to derive a lower-bound value (LB) and derive an upper-bound value (UB) using a rapid branching strategy. The optimal solution is derived by iteratively updating the upper- and lower-bound values. Three acceleration strategies, namely, initial solution iteration, delayed constraints, and column removal, were designed to accelerate the computation. The effectiveness and efficiency of the proposed model and algorithm were tested using Wuhan-Guangzhou high-speed railroad data. The results show that the proposed model and algorithm can quickly reduce the defined cost function by 38.2% and improve the average travel speed by 10.7 km/h, which indicates that our proposed model and algorithm can effectively improve the quality of a constructed train timetable and the travel efficiency for passengers.
Document type: Articl
Developments in British banking: lessons for regulation and supervision
Banks and banking - Great Britain ; Great Britain
Higher plants use LOV to perceive blue light.
Higher plants use several classes of blue light receptors to modulate a wide variety of physiological responses. Among them, both the phototropins and members of the Zeitlupe (ZTL) family use light oxygen voltage (LOV) photosensory domains. In Arabidopsis, these families comprise phot1, phot2 and ZTL, LOV Kelch Protein 2 (LKP2), and Flavin-binding Kelch F-box1 (FKF1). It has now been convincingly shown that blue-light-induced autophosphorylation of the phot1 kinase domain is an essential step in signal transduction. Recent experiments also shed light on the partially distinct photosensory specificities of phot1 and phot2. Phototropin signaling branches rapidly following photoreceptor activation to mediate distinct responses such as chloroplast movements or phototropism. Light activation of the LOV domain in ZTL family members modulates their capacity to interact with GIGANTEA (GI) and their ubiquitin E3 ligase activity. A complex between GI and FKF1 is required to trigger the degradation of a repressor of CO (CONSTANS) expression and thus modulates flowering time. In contrast, light-regulated complex formation between ZTL and GI appears to limit the capacity of ZTL to degrade its targets, which are part of the circadian oscillator
Impact of increasing methyl branches in aromatic hydrocarbons on diesel engine combustion and emissions
Lignocellulosic materials have been identified as potential carbon–neutral sources of sustainable power production. Catalytic conversion of lignocellulosic biomass results in liquid fuels with a variety of aromatic molecules. This paper investigates the combustion characteristics and exhaust emissions of a series of alkylbenzenes, of varying number of methyl branches on the monocyclic aromatic ring, when combusted in a direct injection, single cylinder, compression-ignition engine. In addition, benzaldehyde (an aldehyde (-CHO) branch on the monocyclic ring) was also tested. All the molecules were blended with heptane in different proportions, up to 60% wt/wt. The tests were conducted at a constant engine speed of 1200 rpm, a fixed engine load 4 bar IMEP, and at two injection modes: constant start of fuel injection at 10 CAD BTDC, and varying fuel injection timing to maintain constant start of fuel combustion at TDC. The results showed that the ignition delay period increased with increasing number of methyl branches on the ring, due to the rapid consumption of OH radicals by the alkylbenzenes for oxidation to stable benzyl radicals. Peak heat release rates, and concurrently NOx emissions, initially increased with increasing methyl branches, but subsequently both decreased as the bulk of heat release occurred further into the expansion stroke with significant thermal energy losses. With the exception of toluene, the number of particles in the engine exhaust increased as the number of methyl branches on the aromatic ring increased, attributable to the formation of thermally stable benzyl radicals
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