34555418
Nonmonotonic Reasoning with Multiple Belief Sets
In the present paper we introduce nonmonotonic belief set operators and selection operators to formalize and to analyze multiple belief sets in an abstract setting. We define and investigate formal properties of belief set operators as absorption, congruence, supradeductivity and weak belief monotony. Furthermore, it is shown that for each belief set operator satisfying strong belief cumulativity there exists a largest monotonic logic underlying it, thus generalizing a result for nonmonotonic inference operations. Finally, we study abstract properties of selective inference operations connected to belief set operators and which are used to choose one of the possible views.
9686977
A Temporal Model Theory for Default Logic
By explicitly identifying the temporal aspect of a default rule as it is used in a reasoning process, it is argued that a natural semantic theory of Reiter's default logic is a temporal one. To be able to accommodate the lack of complete knowledge at any point in time, the temporal models should be partial models. A temporal partial logic is introduced, and it is shown that this logic can provide semantics for Reiter's default logic.
3010463
Equality Languages and Fixed Point Languages
This paper considers equality languages and fixed-point languages of homomorphisms and deterministic gsm mappings. It provides some basic properties of these classes of languages. We introduce a new subclass of dgsm mappings, the so-called symmetric dgsm mappings. We prove that (unlike for arbitrary dgsm mappings) their fixed-point languages are regular but not effectively obtainable. This result has various consequences. In particular we strengthen a result from Ehrenfeucht, A., and Rozenberg, G. [(1978), Theor. Comp. Sci. 7, 169–184] by pointing out a class of homomorphisms which includes elementary homomorphisms but still has regular equality languages. Also we show that the result from Herman, G. T., and Walker, A. [(1976), Theor. Comp. Sci. 2, 115–130] that fixed-point languages of DIL mappings are regular, is not effective.
11417476
Look-Ahead on Pushdowns
Abstract The general notion of look-ahead on pushdowns is used to prove that (1) the deterministic iterated pushdown languages are closed under complementation, (2) the deterministic iterated pushdown languages are properly included in the non-deterministic iterated pushdown languages; the counter example is a very simple linear context-free language, independent of the amount of iteration, (3) LL ( k ) iterated indexed grammars can be parsed by deterministic iterated pushdown automata, and (4) it is decidable whether an iterated indexed grammar is LL ( k ). Analogous results hold for iterated pushdown automata with regular look-ahead on the input, and LL -regular iterated indexed grammars.
15984778
XML transformation by tree-walking transducers with invisible pebbles
The pebble tree automaton and the pebble tree transducer are enhanced by additionally allowing an unbounded number of "invisible" pebbles (as opposed to the usual ("visible" ones). The resulting pebble tree automata recognize the regular tree languages (i.e., can validate all generalized DTD's) and hence can find all matches of MSO definable n-ary patterns. Moreover, when viewed as a navigational device, they lead to an XPath-like formalism that has a path expression for every MSO definable binary pattern. The resulting pebbletree transducers can apply arbitrary MSO definable tests to (the observable part of) their configurations, they (still) have a decidable typechecking problem, and they can model the recursion mechanism of XSLT. The time complexity ofthe typechecking problem for conjunctive queries that use MSO definable binary patterns can often be reduced through the use of invisible pebbles.
27165586
A new natural structural congruence in the pi-calculus with replication
Abstract.In the $\pi$-calculus with replication, a new structural congruence called “middle” congruence is investigated: a notion of structural equivalence of processes in which replication of a process is viewed as a potential rather than an actual infinite number of copies of the process, in the sense that copies are spawned at need rather than produced all at once. It is slightly weaker than standard congruence (which is also of the potential type) but stronger than the extended congruence investigated before by the authors (which is of the actual type). It is shown that middle congruence has the same desirable properties as extended congruence: it is decidable and it has a concrete multiset semantics. Thus, these properties do not depend on the distinction between potential and actual replication.
9307461
Context Free Normal Systems and ETOL Systems
Abstract The relationship between extended context free normal systems (a nondeterministic version of Tag systems of Post with deletion number equal to 1 and using nonterminals) and ETOL systems is considered. It is demonstrated that the class of languages generated by context free normal systems (denoted L (ECFN)) lies strictly between the clas of EOL languages (denoted L (EOL)) and the class of ETOL languages. Several characterizations of L (ECFN)) in terms of L (EOL)) are provided and a number of closure properties of L '(ECFN) are established.