Abstract. Given a set of processes and a set of tests on these processes we show how to define in a natural way three different eyuitalences on processes. ThesP equivalences are applied to a particular language CCS. We give associated complete proof systems and fully abstract models. These models have a simple representation in terms of trees.

The time delay as Internet signals cross the Atlantic is about 70 milliseconds, about the same time it takes for a nerve impulse to run from your finger to your brain. Parallels between computation and cognition run as far back as computers themselves. Although at first it feels as if the cold, abstruse, more formal aspects of computation are divorced from the rich ecology of the human–computer interface, the two are intimately bound. Mathematics has also been part of this picture. Indeed the theory of computation predates digital computers themselves as mathematicians pondered the limits of human reasoning and computation. There are a number of aspects of this interplay between computation, mathematics and the human–computer interface. First, understanding your raw material is essential in all design. Part of the material of human–computer interaction is the computer itself. Theoretical and formal aspects of computing can help us understand the practical and theoretical limits of computer systems, and thus design around these limits. Second, diagrams, drawings and models are an integral part of the design process.

Timed automata are governed by an idealized semantics that assumes a perfectly precise behavior of the clocks. The traditional semantics is not robust because the slightest perturbation in the timing of actions may lead to completely different behaviors of the automaton. Following several recent works, we consider a relaxation of this semantics, in which guards on transitions are widened by ∆> 0 and clocks can drift by ε> 0. The relaxed semantics encompasses the imprecisions that are inevitably present in an implementation of a timed automaton, due to the finite precision of digital clocks. We solve the safety verification problem for this robust semantics: given a timed automaton and a set of bad states, our algorithm decides if there exist positive values for the parameters ∆ and ε such that the timed automaton never enters the bad states under the relaxed semantics.

the port with name #,value u, and continuation f as #:hu,fi.Two ports are complementary if their names match, one #plain" and the other #barred." Thus, ports with names # and # are complementary. Complementary ports can be #joined," providing a path for their processes to communicate. Milne and Milner have a graphical notation to illustrate processes and communication paths. Figure 8-1 shows the graphic for process r. This process has two ports, # and #.Port # has value u 1 and continuation f 1 . Similarly, #'s value is u 2 and its continuation, f 2 . When two processes communicate #compose#, they simultaneously exchange information and recon#gure themselves into new processes. Processes communicate through complementary ports. If r<F9.7