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arXiv 0812.2423cs.LO
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2-栈可见下推自动机的表达能力

On the Expressive Power of 2-Stack Visibly Pushdown Automata

Benedikt Bollig

摘要:

可见下推自动机是输入驱动的下推自动机,它可以识别一些非正则上下文无关语言,同时保持了有限自动机的良好闭包和可判定性。具有多个堆栈的可见下推自动机最近被La Torre,Madhusudan和Parlato考虑,他们进一步利用可见性的概念来获得丰富的自动机类,甚至可以表达上下文无关语言之外的属性。同时,它们的自动机在布尔运算下是封闭的,具有可判定的空性和包含性问题,并且在具有额外嵌套结构的词上享有一元二阶逻辑的逻辑表征。这些结果需要一个限制版本的明显下推自动机与多个堆栈,其行为可以分为一个固定数量的阶段。在本文中,我们考虑2-栈可见下推自动机(即,具有两个堆栈的可视下推自动机)以其不受限制的形式。我们表明,他们是表达等价的一元二阶逻辑的存在片段。此外,我们还发现一元二阶量词交替在多重嵌套的词中形成了一个无限的层级结构。结合这些结果,我们得出结论,2-栈可见下推自动机是不封闭的补充。最后,我们讨论了Büchi 2-栈可见下推自动机运行在无限(嵌套)的话的表达能力。通过无限量词扩展逻辑,我们同样可以建立与存在一元二阶逻辑的等价。

英文摘要:

Visibly pushdown automata are input-driven pushdown automata that recognize some non-regular context-free languages while preserving the nice closure and decidability properties of finite automata. Visibly pushdown automata with multiple stacks have been considered recently by La Torre, Madhusudan, and Parlato, who exploit the concept of visibility further to obtain a rich automata class that can even express properties beyond the class of context-free languages. At the same time, their automata are closed under boolean operations, have a decidable emptiness and inclusion problem, and enjoy a logical characterization in terms of a monadic second-order logic over words with an additional nesting structure. These results require a restricted version of visibly pushdown automata with multiple stacks whose behavior can be split up into a fixed number of phases. In this paper, we consider 2-stack visibly pushdown automata (i.e., visibly pushdown automata with two stacks) in their unrestricted form. We show that they are expressively equivalent to the existential fragment of monadic second-order logic. Furthermore, it turns out that monadic second-order quantifier alternation forms an infinite hierarchy wrt words with multiple nestings. Combining these results, we conclude that 2-stack visibly pushdown automata are not closed under complementation. Finally, we discuss the expressive power of Büchi 2-stack visibly pushdown automata running on infinite (nested) words. Extending the logic by an infinity quantifier, we can likewise establish equivalence to existential monadic second-order logic.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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