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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Exploration, exploitation & complexity in biological evolution and self-assembly

Johnston, Iain G. January 2010 (has links)
Self-assembly --- by which ordered structures spontaneously emerge from disordered components --- and biological evolution --- by which Charles Darwin's "endless forms most beautiful" have emerged from the simple chemistry of prehistoric Earth --- may both be pictured as search processes on high-dimensional landscapes, defined respectively by the concepts of energy and fitness. The rich dynamics of these search processes will be studied in order to explain biologically observed features of self-assembly and evolution. This study will introduce and analyse the behaviour of a model for a paradigmatic example of self-assembly, the icosahedral virus capsid, a symmetric structure formed from interacting protein subunits. Results for the thermodynamics and kinetics of model virus assembly will be presented, and the model will be extended to include complicating effects such as different subunit types and cellular crowding. A more general formalism for analysing self-assembling systems will then be introduced and used to suggest a well-defined complexity measure of universal applicability to self-assembled structures. The biological evolution of simple self-assembling structures will be studied using genetic algorithms. The suitability of this modelling approach and its dependence on the many parameters involved will be investigated. Several active areas of enquiry in the field of evolution, including the evolution of complexity, the presence and effect of neutral networks, fluctuations in evolutionary time series and the emergence of symmetry will be investigated within this framework. Throughout this study, we will use the picture of "exploration and exploitation": different approaches to performing optimisation on an unknown landscape, essentially corresponding to a random search and a hill-climbing approach respectively . We will show that, both in self-assembly and evolution, finding an optimal combination of these two approaches gives rise to many of the observed features in this study.
2

Geteiltes Wissen und Retrieval: Ein Prozessmodell zur Unterstützung kollaborativer Suchprozesse

Simon, Lothar, Michel, Kay-Uwe, Schoop, Eric 15 December 2014 (has links) (PDF)
Hoher Wettbewerbsdruck wird für Unternehmen auf allen Ebenen immer stärker spürbar. Noch vor einigen Jahren gab es ganze Bereiche in Wirtschaft, Handel und öffentlicher Verwaltung, die bestenfalls auf eine technisch-inhaltliche Optimierung der jeweiligen Produkte und Leistungen orientiert waren. Dies hat weitgehend einer markt- und wettbewerbsorientierten Ausrichtung Platz gemacht, Innensicht wird durch verstärkte Außensicht ergänzt.
3

Intermittent transport processes on surfaces / Processus de transport intemittent sur surfaces

Phun, Xuan Lan 25 November 2014 (has links)
Comment les protéïnes trouvent-elles leur chemin vers les rares endroits des molécules d’ADN où elles peuvent perpétuer le processus de vie ? De nombreuses études récentes tendent à prouver que seule une dynamique intermittente, c’est à dire à (au moins) deux régimes permet ce processus. L’objet principal de cette thèse est une étude rigoureuse d’un modèle simplifié de dynamique intermittente. Dans ce modèle la molécule alterne des dynamiques browniennes dans le "bulk" et sur la "surface" (i.e. la molécule d’ADN dans l’exemple plus haut) jusqu’à ce qu’elle atteigne sa cible, une petite fenêtre sur la surface: le temps passé par la molécule à la surface est naturellement modélisé comme une variable exponentielle de paramètre λ. Le principal résultat de la thèse est que quels que soient les paramètres, la recherche purement "par le bulk" n’est jamais optimale, ce qui légitime la thèse de la dynamique intermittente. On y caractérise aussi le cas où le temps optimal est atteint pour λ > 0. L’outil mathématique nouveau est l’introduction d’un opérateur autoadjoint et de sa base orthonormée de vecteurs propres. Cette étude permet d’obtenir un développement asymptotique à λ grand du temps moyen d’atteinte de la cible. Par ailleurs, un modèle nouveau est introduit: c’est celui du tore qui porte un paramètre supplémentaire, à savoir son module. Il est montre dans cette thèse que certaines valeurs du module conduisent à prouver que la stratégie intermittente est considérablement meilleure que celle de la pure diffusion dans le bulk. / How do proteins find their way towards the rare places on DNA molecules where they need to go in order to perpetuate the life process ? Many recent works tend to show that only an intermittent dynamics, that is a dynamics with two or more regimes, allows this process. The main goal of this PhD is a rigorous study of a simplified model of intermittent dynamics. In this model the molecule alternates diffusion in the bulk with a different kind of diffusion on the surface until it reaches its target consisting in a small window on the surface. The time spent by the molecule on the surface is naturally modeled as following an exponential law with parameter λ. The main result of this thesis is to show that, whatever the parameters are, a pure bulk strategy is never optimal, thus reinforcing the hypothesis of intermittent dynamics. One also characterizes the case where the optimal timed is attained for λ > 0. The new mathematical tool is the introduction of a self-adjoint operator and the use of its orthonormal basis of eigenvectors. This tool allows to obtain a precise asymptotic behavior of the mean exit time for λ large. Besides that a new geometrical model is developed, called the torus model. This new model carries a new parameter, namely its modulus. It is shown in this thesis that for some values of the modulus the optimized exit time is significantly (allowing experimental checking for instance) shorter than the pure bulk search.
4

Geteiltes Wissen und Retrieval: Ein Prozessmodell zur Unterstützung kollaborativer Suchprozesse

Simon, Lothar, Michel, Kay-Uwe, Schoop, Eric January 2005 (has links)
Hoher Wettbewerbsdruck wird für Unternehmen auf allen Ebenen immer stärker spürbar. Noch vor einigen Jahren gab es ganze Bereiche in Wirtschaft, Handel und öffentlicher Verwaltung, die bestenfalls auf eine technisch-inhaltliche Optimierung der jeweiligen Produkte und Leistungen orientiert waren. Dies hat weitgehend einer markt- und wettbewerbsorientierten Ausrichtung Platz gemacht, Innensicht wird durch verstärkte Außensicht ergänzt.

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