On Prefix Normal Words and Prefix Normal Forms

TL;DR

This paper studies prefix normal words, characterizes their relation to Parikh vectors, analyzes their language complexity, provides enumeration bounds, and explores their properties and open problems.

Contribution

It introduces a characterization of binary words with the same factor Parikh vectors using prefix normal words and analyzes their language and enumeration properties.

Findings

The language of prefix normal words is not context-free.

Bounds on the number of prefix normal words of length n are established.

The generating function for fixed density prefix normal words is rational.

Abstract

A $1$-prefix normal word is a binary word with the property that no factor has more $1$s than the prefix of the same length; a $0$-prefix normal word is defined analogously. These words arise in the context of indexed binary jumbled pattern matching, where the aim is to decide whether a word has a factor with a given number of $1$s and $0$s (a given Parikh vector). Each binary word has an associated set of Parikh vectors of the factors of the word. Using prefix normal words, we provide a characterization of the equivalence class of binary words having the same set of Parikh vectors of their factors. We prove that the language of prefix normal words is not context-free and is strictly contained in the language of pre-necklaces, which are prefixes of powers of Lyndon words. We give enumeration results on $\textit{pnw}(n)$, the number of prefix normal words of length $n$, showing that, for sufficiently large $n$, \[ 2^{n-4 \sqrt{n \lg n}} \le \textit{pnw}(n) \le 2^{n - \lg n + 1}. \] For fixed density (number of $1$s), we show that the ordinary generating function of the number of prefix normal words of length $n$ and density $d$ is a rational function. Finally, we give experimental results on $\textit{pnw}(n)$, discuss further properties, and state open problems.