English

Dissecting power of intersection of two context-free languages

Formal Languages and Automata Theory 2024-02-14 v7 Discrete Mathematics

Abstract

We say that a language LL is \emph{constantly growing} if there is a constant cc such that for every word uLu\in L there is a word vLv\in L with u<vc+u\vert u\vert<\vert v\vert\leq c+\vert u\vert. We say that a language LL is \emph{geometrically growing} if there is a constant cc such that for every word uLu\in L there is a word vLv\in L with u<vcu\vert u\vert<\vert v\vert\leq c\vert u\vert. Given two infinite languages L1,L2L_1,L_2, we say that L1L_1 \emph{dissects} L2L_2 if L2L1=\vert L_2\setminus L_1\vert=\infty and L1L2=\vert L_1\cap L_2\vert=\infty. In 2013, it was shown that for every constantly growing language LL there is a regular language RR such that RR dissects LL. In the current article we show how to dissect a geometrically growing language by a homomorphic image of intersection of two context-free languages. Consider three alphabets Γ\Gamma, Σ\Sigma, and Θ\Theta such that Σ=1\vert \Sigma\vert=1 and Θ=4\vert \Theta\vert=4. We prove that there are context-free languages M1,M2ΘM_1,M_2\subseteq \Theta^*, an erasing alphabetical homomorphism π:ΘΣ\pi:\Theta^*\rightarrow \Sigma^*, and a nonerasing alphabetical homomorphism φ:ΓΣ\varphi : \Gamma^*\rightarrow \Sigma^* such that: If LΓL\subseteq \Gamma^* is a geometrically growing language then there is a regular language RΘR\subseteq \Theta^* such that φ1(π(RM1M2))\varphi^{-1}\left(\pi\left(R\cap M_1\cap M_2\right)\right) dissects the language LL.

Keywords

Cite

@article{arxiv.2006.15160,
  title  = {Dissecting power of intersection of two context-free languages},
  author = {Josef Rukavicka},
  journal= {arXiv preprint arXiv:2006.15160},
  year   = {2024}
}