Counting scattered palindromes in a finite word

Abstract

We investigate the scattered palindromic subwords in a finite word. We start by characterizing the words with the least number of scattered palindromic subwords. Then, we give an upper bound for the total number of palindromic subwords in a word of length $n$ in terms of Fibonacci number $F_n$ by proving that at most $F_n$ new scattered palindromic subwords can be created on the concatenation of a letter to a word of length $n-1$. We propose a conjecture on the maximum number of scattered palindromic subwords in a word of length $n$ with $q$ distinct letters. We support the conjecture by showing its validity for words where $q\ge \frac{n}{2}$.