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 کاملExtending Linear Indexed Grammars
This paper presents a possibility to extend the formalism of linear indexed grammars. The extension is based on the use of tuples of pushdowns instead of one pushdown to store indices during a derivation. If a restriction on the accessibility of the pushdowns is used, it can be shown that the resulting formalisms give rise to a hierarchy of languages that is equivalent with a hierarchy defined ...
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...
متن کاملذخیره در منابع من
با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید
ژورنال
عنوان ژورنال: Theoretical Computer Science
سال: 1984
ISSN: 0304-3975
DOI: 10.1016/0304-3975(84)90023-9