ABSTRACT
The results in Chapter 2 are mainly obtained by S. Hristova, as part of the results in Section 2.1 are published in [Hristova and Stefanova 2010a], and part of the results in Section 2.3 are adopted by [Hristova and Stefanova 2010b]. A part of the results in Section 2.2 are proved by S. Hristova and J. Henderson.
