Acknowledgments

First I would like to thank Mario Pérez Jiménez for his contagious optimism, continuous help in any problem and altruism. I owe him very much. I also grate-fully acknowledge all the members of the Research Group in Natural Computing for creating such a friendly and stimulating scientific atmosphere at University of Seville. I would also like to thank to Gheorghe Păun, my adviser during my doctoral stage in Spain. I am very indebted for his constant support, advice that went beyond the science, and enthusiasm for my research. My thanks are addressed also to Grzegorz Rozenberg for the many constructive scientific discussions and guidance during my research stage in Leiden, The Nether-lands. A special thank I address to my faculty professors Christian Calude and Dan Luca S¸erbanat¸i from whom I have learned the first steps in the theory of computation. I would also like to thank my family for their love and encouragements and especially to my father who always stand on me, backing me in any matter. Last, but not least, I would like to thank my wife Cristina and my son Andrei, for their patience, support and understanding, as well as for other reasons beyond the scope of this dissertation. My most intense collaborations have been with Artiom Alhazov, Ioan Ardelean, Matteo Cavaliere, Agustín Riscos-Núñez, Rodica Ceterchi, Radu Gramatovici, Mihai Ionescu, and Peter Leupold whose clarity, persistence, ability to create new models, and desire to write new publications, have taught me a lot. Drago¸s Sburla