AcknowledgementsScience needs freedom. It needs free thought, it needs free time, it needs free talk. Philippe Blanchard is among those teachers who are deeply committed to such an understanding of science. I am very proud and grateful to be among his students.I am also very grateful to Ricardo Lima. He is probably the person who engaged most in the details of this project and should really be an honorary member of the reading committee. Thank you for always inspiring discussions and the critical reading of all parts of this work.Tanya Araújo has given me encouragement since the first day we met. She also read through all the thesis and her advises (especially during the last turbulent months) helped a lot to finalize the writing.I was once told that those people who are most busy (those that really are and do not just pretend to be) are also the people who always have time when you approach them with some question or meet them on the floor. I don't know whether this is a general rule, but it certainly applies to Dima Volchenkov. Thank you for an open ear whenever I knocked on your door.A special thanks goes to Hanne Litschewsky, our great secretary in E5. Due to Hanne, I could experience how comfortable, how helpful, yet sometimes essential it is to be supported in all the bureaucratic aspects of science.All of this would have been a lot more difficult without the unconditional support of my family. I am very thankful to my parents, my parents-in-law, and especially to my wife Nannette.Financial support of the German Federal Ministry of Education and Research (BMBF) through the project Linguistic Networks is also gratefully acknowledged (http://project.linguistic-networks.net).
AbstractThis thesis introduces a Markov chain approach that allows a rigorous analysis of a class of agent-based models (ABMs). It provides a general framework of aggregation in agent-based and related computational models by making use of Markov chain aggregation and lumpability theory in order to link between the micro and the macro level of observation.The starting point is a microscopic Markov chain description of the dynamical process in complete correspondence with the dynamical behavior of the agent model, which is obtained by considering the set of all possible agent configurations as the state space of a huge Markov chain. This is referred to as micro chain, and an explicit formal representation including microscopic transition rates can be derived for a class of models by using the random mapping representation of a Markov process. The explicit micro formulation enables the application of the theory of Markov chain aggregation -namely, lumpability -in order to reduce the state space of the micro chain and relate microscopic descriptions to a macroscopic formulation of interest. Well-known conditions for lumpability make it possible to establish the cases where the macro model is still Markov, and in this case we obtain a complete picture of the dynamics including the transient stage, the most interesting phase in applications.F...