907 research outputs found
Topological obstructions for vertex numbers of Minkowski sums
We show that for polytopes P_1, P_2, ..., P_r \subset \R^d, each having n_i
\ge d+1 vertices, the Minkowski sum P_1 + P_2 + ... + P_r cannot achieve the
maximum of \prod_i n_i vertices if r \ge d. This complements a recent result of
Fukuda & Weibel (2006), who show that this is possible for up to d-1 summands.
The result is obtained by combining methods from discrete geometry (Gale
transforms) and topological combinatorics (van Kampen--type obstructions) as
developed in R\"{o}rig, Sanyal, and Ziegler (2007).Comment: 13 pages, 2 figures; Improved exposition and less typos.
Construction/example and remarks adde
Comparison of spatial downscaling methods of general circulation model results to study climate variability during the last glacial maximum
The extent to which climate conditions influenced the spatial
distribution of hominin populations in the past is highly debated.
General circulation models (GCMs) and archaeological data have been
used to address this issue. Most GCMs are not currently capable of
simulating past surface climate conditions with sufficiently
detailed spatial resolution to distinguish areas of potential
hominin habitat, however. In this paper, we propose a statistical
downscaling method (SDM) for increasing the resolution of climate
model outputs in a computationally efficient way. Our method uses a
generalised additive model (GAM), calibrated over present-day
climatology data, to statistically downscale temperature and
precipitation time series from the outputs of a GCM simulating the
climate of the Last Glacial Maximum (19âŻ000â23âŻ000âŻBP) over western
Europe. Once the SDM is calibrated, we first interpolate the
coarse-scale GCM outputs to the final resolution and then use the
GAM to compute surface air temperature and precipitation levels
using these interpolated GCM outputs and fine-resolution
geographical variables such as topography and distance from an
ocean. The GAM acts as a transfer function, capturing non-linear
relationships between variables at different spatial scales and
correcting for the GCM biases. We tested three different techniques
for the first interpolation of GCM output: bilinear, bicubic and
kriging. The resulting SDMs were evaluated by comparing downscaled
temperature and precipitation at local sites with paleoclimate
reconstructions based on paleoclimate archives (archaeozoological
and palynological data) and the impact of the interpolation
technique on patterns of variability was explored. The SDM based on
kriging interpolation, providing the best accuracy, was then
validated on present-day data outside of the calibration period. Our
results show that the downscaled temperature and precipitation
values are in good agreement with paleoclimate reconstructions at
local sites, and that our method for producing fine-grained
paleoclimate simulations is therefore suitable for conducting
paleo-anthropological research. It is nonetheless important to
calibrate the GAM on a range of data encompassing the data to be
downscaled. Otherwise, the SDM is likely to overcorrect the
coarse-grain data. In addition, the bilinear and bicubic
interpolation techniques were shown to distort either the temporal
variability or the values of the response variables, while the
kriging method offered the best compromise. Since climate
variability is an aspect of the environment to which human
populations may have responded in the past, the choice of
interpolation technique is therefore an important consideration.</p
Characterizing Protein Conformation Space
In this work, we propose a radical approach for exploring the space of all possible protein structures. We present techniques to explore the clash-free conformation space, which comprises all protein structures whose atoms are not in self-collision. Unlike energy based methods, this approach allows efficient exploration and remains general -- the benefits of characterization of the space apply to all proteins. We hypothesize that this conformation space branches into many small funnels as we sample compact conformations. We develop a compact representation the conformation space, and give experimental results that support our hypothesis. Potential applications of our method include protein folding as well as observing structural relationships between proteins.Singapore-MIT Alliance (SMA
Similar compositional turnover but distinct insular environmental and geographical drivers of native and exotic ants in two oceans
Aim
This study aims to quantify the patterns in compositional turnover of native and exotic ants on small islands in two oceans, and to explore whether such patterns are driven by similar environmental, geographical and potentially biotic variables.
Location
Pacific and Atlantic islands.
Time period
Present.
Major taxa studied
Ants.
Methods
We applied MultiâSite Generalised Dissimilarity Modelling (MSâGDM), which relates zeta diversity, the number of species shared by a given number of islands, to differences in environmental, geographical and biotic drivers. The use of zeta diversity enabled us to differentiate the contribution of rare species (shared by few islands) from those of widespread ones (shared by multiple islands) to compositional turnover. For completion, we also related species richness of insular ants per island with the same set of explanatory variables using Generalised Additive Models (GAM).
Results
Pacific and Atlantic islands have similar patterns of ant species turnover and richness, albeit partly driven by different drivers. Native and exotic species turnover are mostly explained by the same set of variables in the Pacific (annual precipitation and distance to the nearest island), but not in the Atlantic (annual precipitation is a good predictor of native species turnover, but none of the variables considered in our study explained exotic species turnover). No signal of biotic interactions was detected at the insular community level.
Main conclusions
Successful invasion strategies may depend on a combination of factors specific to the region in question. In the Pacific, milder environments and the absence of natives on certain islands enable exotic ants to select the same types of environment as native ants. In the harsher Atlantic Ocean, however, native ant species are likely to be well adapted to local environmental conditions, making it harder for exotics to become established. Exotic ant species, therefore, potentially rely on other attributes to establish, such as a combination of tolerance to a wide range of environmental conditions and humanâmediated colonization
The effect of cross-boundary management on the trajectory to commonness in biological invasions
The number of alien species introduced and undergoing range expansion in novel environments is steadily increasing, with important consequences for native ecosystems. The efficacy of management planning and decision making to limit such invasions can be improved by understanding how interventions will impact the population dynamics of recently introduced species. To do so, here we expand on a typological framework that enables the classification of populations over time into 10 categories of commonness, and apply it to a spatially discrete metapopulation with heterogeneous abundance across spatial units (patches). We use this framework to assess the effect of cross-boundary management on the capacity of a metapopulation with different demographic and dispersal characteristics, including time lags in population growth, to become common. We demonstrate this framework by simulating a simple theoretical metapopulation model capable of exploring a range of environments, species characteristics, and management actions. Management can vary in the efficacy of propagule interception between patches, and in the synchronisation of the implementation of these measures across patches (i.e. if management is implemented simultaneously across patches). Simulations show that poor interception efficacy that only modestly reduces the number of propagules entering a given spatial unit cannot be compensated for by strong management synchronisation between spatial units. Management synchronisation will nonetheless result in a reduction in rates of spread once a critical threshold of interception efficacy has been met. Finally, time lags in population growth that may result in delayed spread are an important aspect to be considered in management as they can amplify the efficacy of management. Our results demonstrate how a typological framework of categories of commonness can be used to provide practical insights for the management of biological invasions
Systematic use of models of concurrency in executable domain-specific modelling languages
Language-Oriented Programming (LOP) advocates designing eXecutable Domain-Specific Modeling Languages (xDSMLs) to facilitate the design, development, verification and validation of modern softwareintensive and highly-concurrent systems. These systems place their needs of rich concurrency constructs at the heart of modern software engineering processes. To ease theirdevelopment, theoretical computer science has studied the use of dedicated paradigms for the specification of concurrent systems, called Models of Concurrency (MoCs). They enable the use of concurrencyaware analyses such as detecting deadlocks or starvation situations, but are complex to understand and master. In this thesis, we develop and extend an approach that aims at reconciling LOP and MoCs by designing so-called Concurrencyaware xDSMLs. In these languages, the systematic use of a MoC is specified at the language level, removing from the end-user the burden of understanding or using MoCs. It also allows the refinement of the language for specific execution platforms, and enables the use of concurrency-aware analyses on the systems
DĂ©veloppement dâun modĂšle centrĂ© sur lâindividu des dĂ©placements du caribou, du loup et de lâorignal, et de leurs interactions, en forĂȘt borĂ©ale amĂ©nagĂ©e
Le caribou forestier est une espĂšce menacĂ©e au Canada, la principale hypothĂšse au dĂ©clin des populations Ă©tant lâintensification de la prĂ©dation provoquĂ©e par les perturbations anthropiques du paysage. Afin de faire face Ă cette situation, il est nĂ©cessaire dâĂ©tudier et comprendre lâimpact de lâenvironnement sur les interactions prĂ©dateur-proies entre le caribou et le loup, ainsi quâavec lâorignal, qui est sa principale proie alternative. Pour cela, cette thĂšse prĂ©sente la conception dâun modĂšle centrĂ© sur lâindividu des dĂ©placements de ces trois espĂšces en fonction de leur environnement, dont rĂ©sulteront les interactions prĂ©dateur-proies. Afin de permettre lâapplication de ce modĂšle sur de longues pĂ©riodes, et donc pour un environnement changeant, une mĂ©thodologie a Ă©tĂ© dĂ©veloppĂ©e, qui sâarticule atour de deux aspects principaux. Tout dâabord, la notion de niveaux dâĂ©mergence est introduite, permettant dâordonner les comportements observables du systĂšme selon leurs interdĂ©pendances, afin de choisir comme trait du modĂšle un com- portement correspondant au domaine dâapplicabilitĂ© visĂ©. Ordonner les comportements selon leurs niveaux dâĂ©mergence permet Ă©galement dâidentifier la redondance entre les patrons, qui peut ĂȘtre Ă lâorigine dâun phĂ©nomĂšne de sur-apprentissage lorsquâils sont utilisĂ©s lors de la calibration. Dans un second temps, un nouveau protocole pour la calibration et la validation du ou des traits choisis Ă lâaide des niveaux dâĂ©mergence, nommĂ© rĂ©plication de systĂšme basĂ© sur lâindividu (Individual Based System Replication - IBSRtion) est Ă©galement prĂ©sentĂ©. Ce protocole met lâemphase sur la modĂ©lisation directe, contrairement au principal protocole existant, la modĂ©lisation orientĂ©e patrons (Pattern Oriented Modelling - POM), et permet une approche empirique en gĂ©nĂ©rant artificiellement des donnĂ©es non disponibles ou ne pouvant ĂȘtre rĂ©coltĂ©es par des Ă©tudes de terrains. IBSRtion a Ă©galement lâavantage de pouvoir ĂȘtre intĂ©grĂ©e dans POM, afin de contribuer Ă la crĂ©ation dâune mĂ©thodologie universelle pour la conception de modĂšles centrĂ©s sur lâindividu. Le processus de conception de ce modĂšle aura entre autre permis de faire une synthĂšse des connaissances et dâidentifier certaines lacunes. Une Ă©tude visant Ă palier le manque de connaissances satisfaisantes sur les rĂ©ponses comportementales Ă court-terme des proies face au risque de prĂ©dation a notamment permis dâobserver que celles-ci sont une combinaison de comportements chroniques et Ă©phĂ©mĂšres, et que les mĂ©canismes qui en sont Ă lâorigine sont complexes et non-linĂ©aires. Le rĂ©sultat de ce travail est un modĂšle complexe utilisant de nombreux sous-modĂšles, et calibrĂ© de façon empirique, applicable Ă une grande variĂ©tĂ© dâenvironnements. Ce modĂšle a permis de tester lâimpact de lâenfeuillement sur les relations prĂ©dateur-proies. Des simulations ont Ă©tĂ© effectuĂ©es pour diffĂ©rentes quantitĂ©s dâenfeuillement, suivant deux configurations spatiales diffĂ©rentes. Les rĂ©sultats de simulation suggĂšrent que des plans dâamĂ©nagement considĂ©rant Ă©galement lâhabitat de lâorignal pourraient ĂȘtre bĂ©nĂ©fiques pour le caribou forestier, car ils permettraient dâamĂ©liorer la sĂ©grĂ©gation spatiale entre les deux espĂšces, et donc entre le caribou et le loup. En le couplant avec un module de naissances et de morts naturelles ainsi quâun modĂšle dâĂ©volution du paysage, ce modĂšle permettra par la suite dâĂ©valuer lâimpact de plans dâamĂ©nagement forestier sur la viabilitĂ© des populations de caribou forestier.Forest-dwelling woodland caribou are considered threatened in Canada. The main hypothesis to the decrease of populations is the intensification of predation provoked by anthropogenic perturbations in the landscape. To deal with this situation, it is necessary to study and to understand the impact of the environment on the predator-prey interactions between the caribou and the wolf, and with the moose, its main alternative prey. In this perspective, this thesis presents the design of an individual-based model of displacements of these three species with respect to their environment, from which the predator-prey interactions will result. To allow for the application of this model for long periods of time, i.e. for changing environments, a methodology has been developed, which is based on two key points. First, the notion of levels of emergence is introduced, allowing to order the different observable comportments of the system according to their inter-dependencies, to help with choosing a trait of the model corresponding to the intended domain of applicability. Ordering the comportments according to their levels of emergence also permits to identify redundancy between patterns, which can lead to over-fitting when they are used for calibration. Second, a new methodology for calibration and validation of the trait(s) chosen by means of the levels of emergence framework is also presented, named Individual Based System Replication (IBSRtion). This protocol emphasizes forward modelling, contrary to the main existing methodology, Pattern Oriented Modelling (POM), and allows to use an empirical approach by artificially generating data that are unavailable or that cannot be obtained by means of field studies. IBSRtion can also be integrated into POM, to contribute to the establish- ment of a universal methodology for the design of individual based models. The design process of this model allowed for a synthesis of existing knowledge and to point out some gaps. More specifically, a study conducted to deal with the lack of sufficient information on the short-term response of prey to predation risk allowed to observe that their behavioural responses to prior presence of predators are a combination of chronic and ephemeral behaviours, and that the mechanisms that produce them are complex and non-linear. The outcome of this work is a complex model, using many sub-models, and calibrated in an empirical fashion, that can be applied to a wide variety of environments. This model allowed to test the impact of the encroachment of deciduous trees on predator-prey relations. Simulations have been run for different quantities of encroachment, according to two different spatial configurations. Simulation results suggest that management plans taking into account the mooseâs habitat might benefit woodland caribou, because they could increase spatial segregation between the two species, and thus between caribou and wolf. Coupling this model with a module of birth and natural death, along with a model of landscape transformation, would allow the assessment of the impact of different forest management plans on the viability of woodland caribou populations
Solving the Direction Field for Discrete Agent Motion
Models for pedestrian dynamics are often based on microscopic approaches
allowing for individual agent navigation. To reach a given destination, the
agent has to consider environmental obstacles. We propose a direction field
calculated on a regular grid with a Moore neighborhood, where obstacles are
represented by occupied cells. Our developed algorithm exactly reproduces the
shortest path with regard to the Euclidean metric.Comment: 8 pages, 4 figure
A Simple Reactive Obstacle Avoidance Algorithm and Its Application in Singapore Harbor
Autonomous surface craft (ASC) are increasingly attractive as a means for performing harbor operations including monitoring and inspection. However, due to the presence of many fixed and moving structures such as pilings, moorings, and vessels, harbor environments are extremely dynamic and cluttered. In order to move autonomously in such conditions ASCâs must be capable of detecting stationary and moving objects and plan their paths accordingly. We propose a simple and scalable online navigation scheme, wherein the relative motion of surrounding obstacles is estimated by the ASC, and the motion plan is modified accordingly at each time step. Since the approach is model-free and its decisions are made at a high frequency, the system is able to deal with highly dynamic scenarios. We deployed ASCâs in the Selat Pauh region of Singapore Harbor to test the technique using a short-range 2-D laser sensor; detection in the rough waters we encountered was quite poor. Nonetheless, the ASCâs were able to avoid both stationary as well as mobile obstacles, the motions of which were unknown a priori. The successful demonstration of obstacle avoidance in the field validates our fast online approach.Massachusetts Institute of Technology. Singapore-MIT Alliance in Research and Technology (SMART
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