Constructions of words rich in palindromes and pseudopalindromes

TL;DR

This paper explores the relationship between classical palindromic words and those rich in both palindromes and pseudopalindromes, introducing a new construction method using an operation S and applying it to generalized Thue–Morse words.

Contribution

It establishes a connection between rich and H-rich words via the operation S and constructs new classes of such words, including applications to generalized Thue–Morse sequences.

Findings

Operation S creates new rich and H-rich words.

Application of S to generalized Thue–Morse words yields new examples.

A link between classical and H-rich words is demonstrated.

Abstract

A narrow connection between infinite binary words rich in classical palindromes and infinite binary words rich simultaneously in palindromes and pseudopalindromes (the so-called $H$-rich words) is demonstrated. The correspondence between rich and $H$-rich words is based on the operation $S$ acting over words over the alphabet $\{0,1\}$ and defined by $S(u_0u_1u_2\ldots) = v_1v_2v_3\ldots$, where $v_i= u_{i-1} + u_i \mod 2$. The operation $S$ enables us to construct a new class of rich words and a new class of $H$-rich words. Finally, the operation $S$ is considered on the multiliteral alphabet $\mathbb{Z}_m$ as well and applied to the generalized Thue--Morse words. As a byproduct, new binary rich and $H$-rich words are obtained by application of $S$ on the generalized Thue--Morse words over the alphabet $\mathbb{Z}_4$.