In the semantics of programming, nite data types such as finite lists, have traditionally been modelled by initial algebras. Later final coalgebras were used in order to deal with in finite data types. Coalgebras, which are the dual of algebras, turned out to be suited, moreover, as models for certain types of automata and more generally, for (transition and dynamical) systems. An important property of initial algebras is that they satisfy the familiar principle of induction. Such a principle was missing for coalgebras until the work of Aczel (Non-Well-Founded sets, CSLI Leethre Notes, Vol. 14, center for the study of Languages and information, Stanford, 1988) on a theory of non-wellfounded sets, in which he introduced a proof principle nowadays called coinduction. It was formulated in terms of bisimulation, a notion originally stemming from the world of concurrent programming languages. Using the notion of coalgebra homomorphism, the definition of bisimulation on coalgebras can be shown to be formally dual to that of congruence on algebras. Thus, the three basic notions of universal algebra: algebra, homomorphism of algebras, and congruence, turn out to correspond to coalgebra, homomorphism of coalgebras, and bisimulation, respectively. In this paper, the latter are taken

Rivest and Lampson have recently introduced SDSI, a Simple Distributed Security Infrastructure. One of the important innovations of SDSI is the use of linked local name spaces. This paper suggests a logical explanation of SDSI's local name spaces, as a complement to the operational explanation given in the SDSI definition.

The aim of this paper is to summarize and analyze some results obtained in 2000-2001 about decidable and undecidable fragments of various first-order temporal logics, give some applications in the field of knowledge representation and reasoning, and attract the attention of the `temporal community' to a number of interesting open problems.

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In this paper we aim to describe dynamic aspects of social and collective attitudes in teams of agents involved in Cooperative Problem Solving (CPS). Particular attention is given to the strongest motivational attitude, collective commitment, and its evolution during team action. First, building on our previous work, a logical framework is sketched in which a number of relevant social and collective attitudes is formalized, leading to the plan-based definition of collective commitments.

This paper is a first attempt towards a theory for reactive planning systems, i.e. systems able to plan and control execution of plans in a partially known and unpredictable environment. We start from an experimental real world application developed at IRST, discuss some of the fundamental requirements and propose a formal theory based on these requirements. The theory takes into account the following facts: (1) actions may fail, since they correspond to complex programs controlling sensors and actuators which have to work in an unpredictable environment; (2) actions need to acquire information from the real world by activating sensors and actuators; (3) actions need to generate and execute plans of actions, since the planner needs to activate different special purpose planners and to execute the resulting plans.

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Intensional HTML addresses the problem of providing a multi-versioned Web site, without the penalties usually associated with large-scale copying and modification of pages (the usual approach). IHTML uses two key ideas. First, the server uses a best-fit algorithm to find the appropriate file when some version of a page is requested. If the exact version requested is not present, the closest match is used. Second, anchors are interpreted as being generic (or intensional, as we will say): an anchor does not link to a particular page, but to a family of versions of a page.

This paper presents a bimodal logic for reasoning about knowledge during knowledge acquisition. One of the modalities represents (effort during) non-deterministic time and the other represents knowledge. The semantics of this logic are tree-like spaces which are a generalization of semantics used for modeling branching time and historical necessity. A finite system of axiom schemes is shown to be canonically complete for the formentioned spaces. A characterization of the satisfaction relation implies the small model property and decidability for this system. 1