Lambda Words: A Class of Rich Words Defined Over an Infinite Alphabet

Lambda words are sequences obtained by encoding the differences between ordered elements of the form i + j theta, where i and j are non-negative integers and 1 < theta < 2. Lambda words are right-infinite words defined over an infinite alphabet that have connections with Sturmian words, Christoffel words, and interspersion arrays. We show that Lambda words are infinite rich words. Furthermore, any Lambda word may be mapped onto a right-infinite word over a three-letter alphabet. Although the mapping preserves palindromes and non-palindromes of the Lambda word, the resulting Gamma word is not rich.