Different Constraints (different + constraint)

Distribution by Scientific Domains

Selected Abstracts

Community Ventures and Access to Markets: The Role of Intermediaries in Marketing Rural Tourism Products

Kathrin Forstner
Many community-based tourism ventures face marketing problems similar to those of other rural producers. They depend on intermediaries, such as private companies, membership organisations, public sector institutions and non-governmental organisations, to facilitate market access. The article analyses the strengths and weaknesses of each type of intermediary, based on different levels of marketing support. Reflecting discussions about marketing assistance in other rural sectors, it argues that intermediary institutions have different areas of expertise and experience different constraints in terms of capacity-building, marketing know-how, financial resources and overall livelihood impacts. Instead of pursuing individual support strategies, it is therefore necessary to develop combined approaches of marketing assistance, depending on location, tourism resources and existing organisational structures. [source]

Phylogenetic analysis of the pearlfish tribe Carapini (Pisces: Carapidae)

ACTA ZOOLOGICA, Issue 4 2000
E. Parmentier
Abstract Fishes of the tribe Carapini (Encheliophis and Carapus) share a noteworthy peculiarity: they shelter in holothurian echinoderms or bivalve hosts. Some species are considered parasitic, others commensal. This study focuses on the phylogeny of the tribe, using two other Carapidae species as an outgroup (Snyderidia canina and Onuxodon fowleri). Insofar as possible, the selected anatomical and behavioural characters where chosen in an ecomorphological perspective, as features that could be responses to various lifestyle-related constraints. Our character selection also took into account the fact that some features are (presumably) linked. Such features were grouped together as a single trait to avoid their overvaluation. This methodology enabled commensals to be separated from parasites, the former belonging to Carapus and the latter to Encheliophis. Carapus species reflect in their morphology the constraints imposed by a diet of hard, mobile, elusive prey, showing predator-type features: a strong dentition, a wide mouth opening, a robust food intake apparatus. On the other hand, the endoparasitic Encheliophis species show a generally weaker buccal apparatus and narrow mouth opening, in relation to the different constraints of their lifestyle where the diet constraints are less pronounced: they eat body parts of their host. Changes in both generic diagnoses are proposed and three species are transferred from Encheliophis to Carapus. [source]

Using DC resistivity tomography to detect and characterize mountain permafrost

Christian Hauck
ABSTRACT Direct-current (DC) resistivity tomography has been applied to different mountain permafrost regions. Despite problems with the very high resistivities of the frozen material, plausible results were obtained. Inversions with synthetic data revealed that an appropriate choice of regularization constraints was important, and that a joint analysis of several tomograms computed with different constraints was required to judge the reliability of individual features. The theoretical results were verified with three field experiments conducted in the Swiss and the Italian Alps. At the first site, near Zermatt, Switzerland, the location and the approximate lateral and vertical extent of an ice core within a moraine could be delineated. On the Murtel rock glacier, eastern Swiss Alps, a steeply dipping boundary at its frontal part was observed, and extremely high resistivities of several M, indicated a high ice content. The base of the rock glacier remained unresolved by the DC resistivity measurements, but it could be constrained with transient EM soundings. On another rock glacier near the Stelvio Pass, eastern Italian Alps, DC resistivity tomography allowed delineation of the rock glacier base, and the only moderately high resistivities within the rock glacier body indicated that the ice content must be lower compared with the Murtel rock glacier. [source]

Pre-handover signalling for QoS aware mobility management

Hakima Chaouchi
In this paper we present a new approach to provide fast handover in Mobile IP. A new Pre-Handover Signalling (PHS) protocol is proposed to allow the network to achieve accurate handover decisions considering different constraints such as QoS, load balancing in the base stations, the user profile, the mobile node service requirements, etc. In addition we propose to minimize the time discovery of the new base station in order to minimize the handover latency. We propose to start the PHS as soon as the mobile node crosses a predefined critical zone area in its current location, this signalling will provide a list of candidate cells to the mobile node with corresponding priorities; the mobile node will select the highest priority base station as soon as the layer two handover occurs. We propose in the current work to use an extension of COPS (Common Open Policy Service) to support the proposed PHS mechanism and overcome the blind handover decisions of Mobile IP and improve the handover performance.,Copyright © 2004 John Wiley & Sons, Ltd. [source]

Heterotachy and Functional Shift in Protein Evolution

IUBMB LIFE, Issue 4-5 2003
Hervé Philippe
Abstract Study of structure/function relationships constitutes an important field of research, especially for modification of protein function and drug design. However, the fact that rational design (i.e. the modification of amino acid sequences by means of directed mutagenesis, based on knowledge of the three-dimensional structure) appears to be much less efficient than irrational design (i.e. random mutagenesis followed by in vitro selection) clearly indicates that we understand little about the relationships between primary sequence, three-dimensional structure and function. The use of evolutionary approaches and concepts will bring insights to this difficult question. The increasing availability of multigene family sequences that has resulted from genome projects has inspired the creation of novel in silico evolutionary methods to predict details of protein function in duplicated (paralogous) proteins. The underlying principle of all such approaches is to compare the evolutionary properties of homologous sequence positions in paralogs. It has been proposed that the positions that show switches in substitution rate over time--i.e., 'heterotachous sites'--are good indicators of functional divergence. However, it appears that heterotachy is a much more general process, since most variable sites of homologous proteins with no evidence of functional shift are heterotachous. Similarly, it appears that switches in substitution rate are as frequent when paralogous sequences are compared as when orthologous sequences are compared. Heterotachy, instead of being indicative of functional shift, may more generally reflect a less specific process related to the many intra- and inter-molecular interactions compatible with a range of more or less equally viable protein conformations. These interactions will lead to different constraints on the nature of the primary sequences, consistently with theories suggesting the non-independence of substitutions in proteins. However, a specific type of amino acid variation might constitute a good indicator of functional divergence: substitutions occurring at positions that are generally slowly evolving. Such substitutions at constrained sites are indeed much more frequent soon after gene duplication. The identification and analysis of these sites by complementing structural information with evolutionary data may represent a promising direction to future studies dealing with the functional characterization of an ever increasing number of multi-gene families identified by complete genome analysis. IUBMB Life, 55: 257-265, 2003 [source]

Constrained least squares methods for estimating reaction rate constants from spectroscopic data

Sabina Bijlsma
Abstract Model errors, experimental errors and instrumental noise influence the accuracy of reaction rate constant estimates obtained from spectral data recorded in time during a chemical reaction. In order to improve the accuracy, which can be divided into the precision and bias of reaction rate constant estimates, constraints can be used within the estimation procedure. The impact of different constraints on the accuracy of reaction rate constant estimates has been investigated using classical curve resolution (CCR). Different types of constraints can be used in CCR. For example, if pure spectra of reacting absorbing species are known in advance, this knowledge can be used explicitly. Also, the fact that pure spectra of reacting absorbing species are non-negative is a constraint that can be used in CCR. Experimental data have been obtained from UV-vis spectra taken in time of a biochemical reaction. From the experimental data, reaction rate constants and pure spectra were estimated with and without implementation of constraints in CCR. Because only the precision of reaction rate constant estimates could be investigated using the experimental data, simulations were set up that were similar to the experimental data in order to additionally investigate the bias of reaction rate constant estimates. From the results of the simulated data it is concluded that the use of constraints does not result self-evidently in an improvement in the accuracy of rate constant estimates. Guidelines for using constraints are given. Copyright © 2002 John Wiley & Sons, Ltd. [source]

The role of water abundance, thermoregulation, perceived predation risk and interference competition in water access by African herbivores

Marion Valeix
Abstract In African savannas, surface water can become limiting and an understanding of how animals address the trade-offs between different constraints to access this resource is needed. Here, we describe water access by ten African herbivore species in Hwange National Park, Zimbabwe, and we explore four possible determinants of the observed behaviours: water abundance, thermoregulation, perceived predation risk and interference competition. On average, herbivores were observed to drink in 80% of visits to a waterhole. The probability of drinking was higher in 2003 (474 mm) than in 2004 (770 mm), and at the end of the dry season than at its beginning. For larger species, this probability may also be related to risks of interference competition with elephants or other herbivores. For smaller species, this probability may also be related to the perceived risk of predation. We also investigate the time spent accessing water to drink. The influence of herd size and the presence of young on the time spent accessing water for most species suggests that perceived predation risk plays a role. Thermoregulation also affects this time: during the hottest periods, herbivores spend less time in open areas, unless when wind is strong, probably owing to evapotranspired heat loss. Résumé Dans les savanes africaines, l'eau de surface disponible peut devenir un facteur limitant et il est nécessaire de comprendre comment les animaux agissent face aux différentes contraintes que pose l'accès à cette ressource. Nous décrivons ici l'accès à l'eau de dix herbivores africains du Parc National de Hwange, au Zimbabwe, et nous explorons quatre facteurs qui sont peut-être déterminants dans les comportements observés: l'abondance de l'eau, la thermorégulation, le risque de prédation ressenti et la compétition/ interférence. En moyenne, on a observé que les herbivores buvaient lors de 80% de leurs visites au point d'eau. La probabilité qu'ils boivent étai plus forte en 2003 (474 mm) qu'en 2004 (770 mm), et à la fin de la saison sèche qu'au début. Pour les plus grandes espèces, cette probabilité pourrait aussi être liée aux risques de compétition par interférence avec les éléphants ou d'autres herbivores. Pour les plus petites espèces, cette probabilité pourrait aussi être liée au risque de prédation ressenti. Nous avons aussi étudié le temps passéà se rendre au point d'eau pour y boire. L'influence de la taille du groupe et de la présence de jeunes sur le temps pris par la plupart des espèces pour se rendre au point d'eau laisse penser que la perception du risque de prédation joue un rôle. La thermorégulation affecte aussi cette durée: pendant les périodes les plus chaudes, les herbivores passent moins de temps dans les espaces ouverts, sauf si le vent est fort, probablement à cause de la perte de chaleur par évapotranspiration. [source]

Material handling device selection in cellular manufacturing

Marcello Braglia
Abstract This paper presents a new multi-criteria decision model for the material handling device (MHD) selection problem in cellular manufacturing systems. Given a set of manufacturing cells based on several automatic work-centres, the technique makes it possible to select a particular MHD for each cell in an integrated way, with different constraints being taken into consideration. The approach is based on two different multi-attribute analyses executed with analytic hierarchy process (AHP) methodology, and a final integer linear programming including important limitations faced by the designer when making MHD investment decisions. An example using real data is provided to illustrate this methodology. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Adversarial models for priority-based networks,

C. Àlvarez
Abstract In this article, we propose several variations of the adversarial queueing model and address stability issues of networks and protocols in those proposed models. The first such variation is the priority model, which is directed at static network topologies and takes into account the case in which packets can have different priorities. Those priorities are assigned by an adversary at injection time. A second variation, the variable priority model, is an extension of the priority model in which the adversary may dynamically change the priority of packets at each time step. Two more variations, namely the failure model and the reliable model, are proposed to cope with dynamic networks. In the failure and reliable models the adversary controls, under different constraints, the failures that the links of the topology might suffer. Concerning stability of networks in the proposed adversarial models, we show that the set of universally stable networks in the adversarial model remains the same in the priority, variable priority, failure, and reliable models. From the point of view of protocols (or queueing policies), we show that several protocols that are universally stable in the adversarial queueing model remain so in the priority, failure, and reliable models. However, we show that the longest-in-system (LIS) protocol, which is universally stable in the adversarial queueing model, is not universally stable in any of the other models we propose. Moreover, we show that no queueing policy is universally stable in the variable priority model. Finally, we analyze the problem of deciding stability of a given network under a fixed protocol. We provide a characterization of the networks that are stable under first-in-first-out (FIFO) and LIS in the failure model (and therefore in the reliable and priority models). This characterization allows us to show that the stability problem under FIFO and LIS in the failure model can be solved in polynomial time. © 2004 Wiley Periodicals, Inc. NETWORKS, Vol. 45(1), 23,35 2005 [source]

Children's work, earnings, and nutrition in urban Mexican shantytowns

Alexandra Brewis
For many children living in conditions of urban poverty, earning money can provide additional resources to them and their families, and this raises interesting questions about the potential biological consequences (costs and benefits) of children's work in ,modern' settings. This study uses time allocation, ethnographic, dietary, and anthropometric data collected with 96 urban Mexican shantytown children (aged 8,12 years) and their older and younger siblings (aged 1,18 years) to test hypotheses related to the effects of children's cash earning and cash contributions to their households for their own and their sibs' nutritional status. Regression models show that children's contributions to household income and the time they allocate to working outside the home makes no difference to their own or their younger siblings' nutritional status assessed anthropometrically. Dietary quality, based on food recalls, is worse in working than non-working children, even taking household income into account. Children's allocation of time to work and their cash contributions to the household do however significantly improve the weight of their older siblings, especially sisters. This suggests children's work in urban ecologies might have different constraints and opportunities for their own and siblings' growth and nutrition than typically observed in subsistence settings. Am. J. Hum. Biol. 2010. © 2009 Wiley-Liss, Inc. [source]

In situ remediation of MTBE utilizing ozone

REMEDIATION, Issue 1 2002
Jeffrey C. Dey
There has been a great deal of focus on methyl tertiary butyl ether (MTBE) over the past few years by local, state, and federal government, industry, public stakeholders, the environmental services market, and educational institutions. This focus is, in large part, the result of the widespread detection of MTBE in groundwater and surface waters across the United States. The presence of MTBE in groundwater has been attributed primarily to the release from underground storage tank (UST) systems at gasoline service stations. MTBE's physical and chemical properties are different than other constituents of gasoline that have traditionally been cause for concern [benzene, toluene, ethylbenzene, and xylenes (BTEX)]. This difference in properties is why MTBE migrates differently in the subsurface environment and exhibits different constraints relative to mitigation and remediation of MTBE once it has been released to subsurface soils and groundwater. Resource Control Corporation (RCC) has accomplished the remediation of MTBE from subsurface soil and groundwater at multiple sites using ozone. RCC has successfully applied ozone at several sites with different lithologies, geochemistry, and concentrations of constituents of concern. This article presents results from several projects utilizing in situ chemical oxidation with ozone. On these projects MTBE concentrations in groundwater were reduced to remedial objectives usually sooner than anticipated. © 2002 Wiley Periodicals, Inc. [source]

Evolution of the special senses in primates: Past, present, and future

Nathaniel J. Dominy
Abstract The present special issue of The Anatomical Record is the result of a symposium entitled Evolution of the Special Senses in Primates. Considered together, the special senses of primates are remarkable because they constitute a singular and definitive suite of mammalian characteristics. Examining their evolution is pivotal for understanding the origin and present-day variation of primate behavior and ecology. Accordingly, the 14 articles assembled here consider the different constraints and opportunities associated with the uptake and use of physical and chemical stimuli. The present issue brings together experts on different primate sensory modalities and stresses events at the sensory periphery, where the organism is exposed to and comes into contact with its environment. Key topics include color vision, the genetics of olfaction, the morphological basis and significance of chemical communication, and the neural organization and scaling of primate sensory systems. The result is a special issue that both reflects our current understanding of primate sensory modalities and challenges certain fundamental assumptions concerning their evolution. © 2004 Wiley-Liss, Inc. [source]

Rapid diversification of mating systems in ciliates

Ciliates are a diverse group of microbial eukaryotes that exhibit tremendous variety in several aspects of their mating systems. To understand the evolutionary forces driving mating system diversification in ciliates, we use a comparative approach synthesizing data from many ciliate species in light of recent phylogenetic analyses. Specifically, we investigate the evolution of number of mating types, mode of mating type inheritance, and the molecular determinants of mating types across the taxonomic diversity of ciliates, with an emphasis on three well-studied genera: Tetrahymena, Paramecium, and Euplotes. We find that there have been many transitions in the number of mating types, and that the requirement of nuclear reorganization may be a more important factor than genetic exchange in determining the optimum number of mating types in a species. We also find that the molecular determinants of mating types and mode of inheritance are evolving under different constraints in different lineages of ciliates. Our results emphasize the need for further detailed examination of mating systems in understudied ciliate lineages. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187,197. [source]