一句话:证明在 ω-字上添加含中性字母的非 ω-正则语言会使 MSO 扩展不可判定。
论文研究 ω-字上一元二阶逻辑(MSO)的扩展问题:若向该逻辑加入一个非 ω-正则语言 L,所得逻辑是否仍可判定。作者证明,只要 L 含有中性字母(neutral letter),则该扩展逻辑必然是不可判定的。这一结论不依赖于 L 的具体结构,而只取决于中性字母这一语言性质,由此给出了强不可判定性界限。作为推论,ω-字上的 ω-正则语言是唯一同时可判定且对布尔运算封闭的 full trio。该工作为 ω-正则性的逻辑刻画与扩张极限提供了清晰边界。
We consider extensions of monadic second-order logic over ω-words, which are obtained by adding one language that is not ω-regular. We show that if the added language L has a neutral letter, then the resulting logic is necessarily undecidable. A corollary is that the ω-regular languages are the only decidable Boolean-closed full trio over ω-words.