一句话：提出双指数空间算法判定双向转换器实现的函数是否可由单向转换器实现，并扩展至扫掠转换器。
双向转换器（等价于流式转换器和 MSO 转换）所实现的函数式单词到单词转换是“正则”映射的自然且标准概念。此前 LICS'13 已证明：判定一个这样的转换能否由某个单向转换器实现是可判定的，但所给算法具有非初等复杂度。本文给出风格不同的算法解决同一问题，其空间复杂度为双指数，从而将其纳入初等复杂度范围。作者进一步将该技术应用于判定双向转换器实现的转换能否由扫掠转换器实现，包括扫掠遍数已知和未知两种情形。该结果澄清了正则单词转换的可实现性与复杂度边界。
Functional transductions realized by two-way transducers (equivalently, by streaming transducers and by MSO transductions) are the natural and standard notion of “regular” mappings from words to words. It was shown recently (LICS'13) that it is decidable if such a transduction can be implemented by some one-way transducer, but the given algorithm has non-elementary complexity. We provide an algorithm of different flavor solving the above question, that has double exponential space complexity. We further apply our technique to decide whether the transduction realized by a two-way transducer can be implemented by a sweeping transducer, with either known or unknown number of passes.