Morphic words, Beatty sequences and integer images of the Fibonacci language

Abstract

Morphic words are letter-to-letter images of fixed points $x$ of morphisms on finite alphabets. There are situations where these letter-to-letter maps do not occur naturally, but have to be replaced by a morphism. We call this a decoration of $x$. Theoretically, decorations of morphic words are again morphic words, but in several problems the idea of decorating the fixed point of a morphism is useful. We present two of such problems. The first considers the so called $AA$ sequences, where $\alpha$ is a quadratic irrational, $A$ is the Beatty sequence defined by $A(n)=\lfloor \alpha n\rfloor$, and $AA$ is the sequence $(A(A(n)))$. The second example considers homomorphic embeddings of the Fibonacci language into the integers, which turns out to lead to generalized Beatty sequences with terms of the form $V(n)=p\lfloor \alpha n\rfloor+qn+r$, where $p,q$ and $r$ are integers.