منابع مشابه
Downward Closures of Indexed Languages
ions. A fruitful idea in the analysis of complex systems is that of abstractions: Instead of working with the original model, one considers a model that has a simpler structure but preserves pertinent properties. A prominent example of such an abstraction is the Parikh image, which is available whenever the semantics of a model is given as a formal language. If L is a language over an alphabet ...
متن کاملIndexed Languages and Unification Grammars
T h e occu rren ce o f purely syn tactica l cross-serial depen den cies in S w iss-G erm an show s that con text-free gram m ars can n ot describe the string sets o f natu ral lan guage [Shi85]. T h e least class in the C h om sk y hierarchy th at can d escribe u n lim ited cross-serial depen dencies is in dexed g ram m ars [A h o 68]. G azd a r discuss in [G a z88] the ap p licab ility o f in...
متن کاملBounded Counter Languages
We show that deterministic finite automata equipped with k two-way heads are equivalent to deterministic machines with a single two-way input head and k− 1 linearly bounded counters if the accepted language is strictly bounded, i.e., a subset of a∗1a ∗ 2 · · · a ∗ m for a fixed sequence of symbols a1, a2, . . . , am. Then we investigate linear speed-up for counter machines. Lower and upper time...
متن کاملDyck-based characterizations of Indexed Languages
Indexed languages are languages recognized by pushdown automata of level 2 and by indexed grammars. We propose here some new characterizations linking indexed languages to context-free languages: the class of indexed languages is the image of the Dyck language by a nice class of context-free transducers, it is also the class of images, by a projection defined by an FO-formula of nested words la...
متن کاملOn Infinite Words Determined by Indexed Languages
We characterize the infinite words determined by indexed languages. An infinite language L determines an infinite word α if every string in L is a prefix of α. If L is regular or context-free, it is known that α must be ultimately periodic. We show that if L is an indexed language, then α is a morphic word, i.e., α can be generated by iterating a morphism under a coding. Since the other directi...
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ژورنال
عنوان ژورنال: RAIRO - Theoretical Informatics and Applications
سال: 1992
ISSN: 0988-3754,1290-385X
DOI: 10.1051/ita/1992260100931