We prove the undecidability of Core XPath 1.0 (CXP) [G. Gottlob and C. Koch,inProc. of 17th Ann. IEEE Symp. on Logic in Computer Science,LICS ’02(Copenhagen,July 2002). IEEE CS Press (2002) 189–202.] extended with anInflationary Fixed Point (IFP) operator. More specifically,we provethat the satisfiability problem of this language is undecidable. In fact,the fragment ofCXP+IFP containing only the self and descendant axes is already undecidable.

We describe a sequent calculus μLJ with primitives for inductive andcoinductive datatypes and equip it with reduction rules allowing a sound translation ofGödel’s system T. We introduce the notion of a μ-closedcategory, relying on a uniform interpretation of open μLJformulas as strong functors. We show that any μ-closed category is asound model for μLJ. We then turn to the construction of a concreteμ-closed category based on Hyland-Ong game semantics. The model relieson three main ingredients: the construction of a general class of strong functors calledopen functors acting on the category of games and strategies, thesolution of recursive arena equations by exploiting cycles in arenas, andthe adaptation of the winning conditions of parity games to build initial algebras andterminal coalgebras for many open functors. We also prove a weak completeness result forthis model, yielding a normalisation proof for μLJ.

A standard bridge between automata theory and logic is provided by the notion ofcharacteristic formula. This paper investigates this problem for the class ofevent-recording automata (ERA), a subclass of timed automata in which clocks areassociated with actions and that enjoys very good closure properties. We first study theproblem of expressing characteristic formulae for ERA in Event-Recording Logic (ERL ), alogic introduced by Sorea to express event-based timed specifications. We prove that theconstruction proposed by Sorea for ERA without invariants is incorrect. More generally, weprove that timed bisimilarity cannot in general be expressed in ERL for the class of ERA ,and study under which conditions on ERA it can be. Then, we introduce the logicWTμ , a new logic for event-based timed specificationscloser to the timed logic ℒν that was introduced byLaroussinie, Larsen and Weise. We prove that it is strictly more expressive than ERL , andthat its model-checking problem against ERA is EXPTIME -complete. Finally, we providecharacteristic formulae constructions in WTμ forcharacterizing the general class of ERA up to timed (bi)similarity and study thecomplexity issues.

We consider μ-calculus formulas in a normal form: after a prefix offixed-point quantifiers follows a quantifier-free expression. We are interested in theproblem of evaluating (model checking) such formulas in a powerset lattice. We assume thatthe quantifier-free part of the expression can be any monotone function given by ablack-box – we may only ask for its value for given arguments. As a first result we provethat when the lattice is fixed, the problem becomes polynomial (the assumption about thequantifier-free part strengthens this result). As a second result we show that anyalgorithm solving the problem has to ask at least about n2(namely Ω(n2/log n)) queries to the function, even when the expressionconsists of one μ and one ν (the assumption about thequantifier-free part weakens this result).

We study iteration and recursion operators in the denotational semantics of typedλ-calculi derived from the multiset relational model of linear logic.Although these operators are defined as fixpoints of typed functionals, we prove themfinitary in the sense of Ehrhard’s finiteness spaces.