一句话:证明存在非 FC-可定义的正则语言,并给出 FC-可定义正则语言的可判定刻画与表达式形式。
论文研究一阶逻辑 FC 可定义的语言类,其中 FC 可借助连接量词考察有限词的所有因子,并已能定义如 ww 之类的非正则语言。作者首先证明并非所有正则语言都能由 FC 定义,从而区分 FC-可定义语言与正则语言。随后,他们从代数、自动机和正则表达式三个角度给出 FC-可定义正则语言的精确且可判定的特征刻画;在表达式层面,该刻画可自然简洁地表述为:在无星广义正则表达式上加入 terminal words 的 Kleene 星。该结果明确了 FC 在正则语言范围内的表达能力,并提供了判定某正则语言是否 FC-可定义的途径。
FC is a first-order logic that reasons over all factors of a finite word using concatenation, and can define non-regular languages like that of all squares (ww). In this paper, we establish that there are regular languages that are not FC-definable. Moreover, we give a decidable characterization of the FC-definable regular languages in terms of algebra, automata, and regular expressions. The latter of which is natural and concise: Star-free generalized regular expressions extended with the Kleene star of terminal words.