We study two important extensions of first-order logic (FO) with iteration, the fixpoint and while queries. The main result of the paper concerns the open problem of the relationship between fixpoint and while: they are the same iff ptime = pspace. These and other expressibility results are obtained using a powerful normal form for while which shows that each while computation over an unordered domain can be reduced to a while computation over an ordered domain via a fixpoint query. The fixpoint query computes an equivalence relation on tuples which is a congruence with respect to the rest of the computation. The same technique is used to show that equivalence of tuples and structures with respect to FO formulas with bounded number of variables is definable in fixpoint. Generalizing fixpoint and while, we consider more powerful languages which model arbitrary computation interacting with a database using a finite set of FO queries. Such computation is modeled by a relational machine...

Motivated by problems that arise in computing degrees of belief, we consider the problem of computing asymptotic conditional probabilities for first-order sentences. Given first-order sentences ' and `, we consider the structures with domain f1; : : : ; Ng that satisfy `, and compute the fraction of them in which ' is true. We then consider what happens to this fraction as N gets large. This extends the work on 0-1 laws that considers the limiting probability of first-order sentences, by considering asymptotic conditional probabilities. As shown by Liogon'kii [Lio69], if there is a non-unary predicate symbol in the vocabulary, asymptotic conditional probabilities do not always exist. We extend this result to show that asymptotic conditional probabilities do not always exist for any reasonable notion of limit. Liogon'kii also showed that the problem of deciding whether the limit exists is undecidable. We analyze the complexity of three problems with respect to this limit: deciding whether it is well-defined, whether it exists, and whether it lies in some nontrivial interval. Matching upper and lower bounds are given for all three problems, showing them to be highly undecidable.

We review recent work in the field of generalized quantifiers on finite models. We give an idea of the methods that are available in this area. Main emphasis is on definability issues, such as whether there is a logic for the PTIME properties of unordered finite models. 1

Sets, functions and relations are powerful structures for modeling software systems. Relational specifications, built from these constructs, are the most common form of formal specification for software systems. However, in sharp contrast to other formal notations, there is an almost complete lack of automated tools for analyzing relational specifications. A method for solving relational formulae must be at the core of any tool for analyzing relational specifications. In this this thesis, I am developing a method for efficiently solving relational formulae, which I call selective enumeration. Selective enumeration uses a generate-and-test approach, but it prevents the generation of the vast majority of cases, which can be considered duplicates. I have implemented four selective enumeration techniques: isomorph elimination, bounded generation, short circuiting and derived variable construction. In this thesis I will provide a firm theoretical and empirical framework for underst...