Completeness is an ideal, although uncommon, feature of abstract interpretations, formalizing the intuition that, relatively to the properties encoded by the underlying abstract domains, there is no loss of information accumulated in abstract computations. Thus, complete abstract interpretations can be rightly understood as optimal. We deal with both pointwise completeness, involving generic semantic operations, and (least) fixpoint completeness. Completeness and fixpoint completeness are shown to be properties that depend on the underlying abstract domains only. Our primary goal is then to solve the problem of making abstract interpretations complete by minimally extending or restricting the underlying abstract domains. Under the weak and reasonable hypothesis of dealing with continuous semantic operations, we provide constructive characterizations; for the least complete extensions and the greatest complete restrictions of abstract domains. As far as fixpoint completeness is concerned, for merely monotone semantic operators, the greatest restrictions of abstract domains are constructively characterized, while it is shown that the existence of least extensions of abstract domains cannot be, in general, guaranteed, even under strong hypotheses. These methodologies, which in finite settings give rise to effective algorithms, provide advanced formal tools for manipulating and comparing abstract interpretations, useful both in static program analysis and in semantics design. A number of examples illustrating these techniques are given

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In the context of Cousot and Cousot's abstract interpretation theory, we present a general framework to define, study and handle operators modifying abstract domains. In particular, we introduce the notions of operators of refinement and compression of abstract domains: A refinement enhances the precision of an abstract domain; a compression operator (compressor) can exist relatively to a given refinement, and it simplifies as much as possible a domain of input for that refinement. The adequateness of our framework is shown by the fact that most of the existing operators on abstract domains fall in it. A precise relationship of adjunction between refinements and compressors is also given, justifying why compressors can be understood as inverses of refinements

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