Abstract
A two-way nondeterministic finite transducer (2-NFT) is a finite automaton with a two-way input tape and a one-way output tape. The generated language of a 2-NFT is the set of all strings it can output (across all inputs). Whereas two-way nondeterministic finite acceptors (2-NFAs) accept only regular languages, 2-NFTs can generate languages which are not even context-free, e.g. \(\{\texttt{a}^n \texttt{b}^n \texttt{c}^n \mid n \geq 0\}\). We prove a pumping lemma for 2-NFT languages which strengthens and generalizes previous results. Our pumping lemma states that every 2-NFT language L is k-iterative for some k ≥ 1. That is, every string in L above a certain length can be expressed in the form x 1 y 1 x 2 y 2 ⋯ x k y k x k + 1, where the ys can be “pumped” to produce new strings in L of the form \(x_1 y_1^i x_2 y_2^i \dotsm x_k y_k^i x_{k+1}\).
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Smith, T. (2014). A Pumping Lemma for Two-Way Finite Transducers. In: Csuhaj-Varjú, E., Dietzfelbinger, M., Ésik, Z. (eds) Mathematical Foundations of Computer Science 2014. MFCS 2014. Lecture Notes in Computer Science, vol 8634. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-44522-8_44
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DOI: https://doi.org/10.1007/978-3-662-44522-8_44
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