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A concurrent Abstract State Machine (ASM) is a family of agents each equipped with a sequential ASM to execute. We define the semantics of concurrent ASMs by concurrent ASM runs which overcome the problems of Gurevich's distributed ASM runs and generalize Lamport's sequentially consistent runs. A postulate characterizing an intuitive understanding of concurrency is formulated. It allows us to state and prove an extension of the sequential ASM thesis to a concurrent ASM thesis.

A behavioural theory consists of machine-independent postulates characterizing a particular class of algorithms or systems, an abstract machine model that provably satisfies these postulates, and a rigorous proof that any algorithm or system stipulated by the postulates is captured by the abstract machine model. The class of interest in this article is that of synchronous parallel algorithms. For this class a behavioural theory has already been developed by Blass and Gurevich, which unfortunately, though mathematically correct, fails to be convincing, as it is not intuitively clear that the postulates really capture the essence of (synchronous) parallel algorithms.In this article we present a much simpler (and presumably more convincing) set of four postulates for (synchronous) parallel algorithms, which are rather close to those used in Gurevich's celebrated sequential ASM thesis, i.e. the behavioural theory of sequential algorithms. The key difference is made by an extension of the bounded exploration postulate using multiset comprehension terms instead of ground terms formulated over the signature of the states. In addition, all implicit assumptions are made explicit, which amounts to considering states of a parallel algorithm to be represented by meta-finite first-order structures.The article first provides the necessary evidence that the axiomatization Preprint submitted to Theoretical Computer ScienceDecember 12, 2017 presented in this article characterizes indeed the whole class of deterministic, synchronous, parallel algorithms, then formally proves that parallel algorithms are captured by Abstract State Machines (ASMs). The proof requires some recourse to methods from finite model theory, by means of which it can be shown that if a critical tuple defines an update in some update set, then also every other tuple that is logically indistinguishable defines an update in that update set.

One common usage of the WWW is t o provide informa-tion services, e.g. for regional event calenders, university lecture management, travel information, etc. Their design requires a clear picture of the system's usage and the supported technical environment. The core of the problem is to provide flexible access units based on extended views o n some underlying database. The extension consists in escort information, links, operational facilities and means for the generation of diflerent presentations. W e shall call such units media objects. In order t o allow also a flexible presentation for them, we adapt the metaphor of a container. I n this way it is possible t o present the same information in various ways depending on the user' profile and the possible technical restrictions of the display device.