Concatenations of Terms of an Arithmetic Progression

Florian Luca, Bertrand Teguia Tabuguia

Published 2022-01-14, updated 2024-05-14Version 2

Let $\left(u(n)\right)_{n\in\mathbb{N}}$ be an arithmetic progression of natural integers in base $b\in\mathbb{N}\setminus \{0,1\}$. We consider the following sequences: $s(n)=\overline{u(0)u(1)\cdots u(n) }^b$ formed by concatenating the first $n+1$ terms of $\left(u(n)\right)_{n\in\mathbb{N}}$ in base $b$ from the right; $s_r(n) = \overline{u(n)u(n-1)\cdots u(0)}^b$; and $\left(s_*(n)\right)_{n\in\mathbb{N}}$, given by $s_*(0)=u(0)$, $s_*(n)=\overline{s_r(n-1)s(n)}^b, n\geq 1$. We construct explicit formulas for these sequences and use basic concepts of linear difference operators to prove they are not P-recursive (holonomic). We also present an alternative proof that follows directly from their definitions. We implemented $\left(s(n)\right)_{n\in\mathbb{N}}$ and $\left(s_r(n)\right)_{n\in\mathbb{N}}$ in the decimal base when $(u(n))_{n\in\mathbb{N}}=\mathbb{N}\setminus \{0\}$.