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- Advances on belief base dynamicsPublication . Garapa, Marco Paulo Ferreirinha; Fermé, Eduardo Leopoldo; Reis, Maurício Duarte LuísThe main goal underlying the research area of belief change consists in finding appropriate ways of modelling the belief state of a rational agent and, additionally, the changes which occur in such a state when the agent receives new information. The most important model of belief change is the so-called AGM model, proposed in [AGM85]. In this model, the belief state of an agent is represented by a belief set—a deductively closed set of sentences. A change consists in adding or removing a specific sentence from a belief set to obtain a new belief set. Two of the main shortcomings pointed out to the AGM model of belief change are the use of belief sets to represent belief states and the (unrealistic) acceptance of any new piece of information. In this thesis we address both those issues. We present axiomatic characterizations for ensconcement-based contractions and for brutal contractions, two kinds of belief bases contraction operators introduced in [Wil94b] that are based on the concept of ensconcement, which is a generalization to the case of belief bases of the concept of epistemic entrenchment introduced in [Ga¨r88, GM88]. We compare the axiomatic characterizations of these operators with those of other well-known base contraction operators and study the interrelations among the former and the contraction operators based on epistemic entrenchments. We study non-prioritized base change operators, namely shielded base contractions and credibility-limited base revisions. We propose several different classes of shielded base contractions and obtain axiomatic characterizations for each one of them. Additionally we thoroughly investigate the interrelations (in the sense of inclusion) among all those classes. Afterwards we perform a similar study for credibility-limited base revisions. Finally, we study the interrelation between the different proposed classes of operators of credibility-limited base revision and of shielded contraction by means of the consistency-preserving Levi identity and the Harper identity.