Sparse subsets of the natural numbers and Euler's totient function

Abstract

In this article, we investigate sparse subsets of the natural numbers and study the sparseness of some sets associated with the Euler's totient function $\varphi$ via the property of `Banach Density'. These sets related to the totient function are defined as follows: $V:=\varphi (\mathbb{N})$ and $N_i:=\{N_i(m):m\in V\}$ for $i=1,2,3,$ where $N_1(m)=\max \{x\in \mathbb{N}:\varphi (x)\le m\}$, $N_2(m)=\max ({\varphi }^{-1}(m))$ and $N_3(m)=\min ({\varphi }^{-1}(m))$ for $m\in V$. Masser and Shiu call the elements of $N_1$ as `sparsely totient numbers' and construct an infinite family of these numbers. Here we construct several infinite families of numbers in $N_2\setminus N_1$ and an infinite family of composite numbers in $N_3$. We also study (i) the ratio $\frac{N_2(m)}{N_3(m)}$, which is linked to the Carmichael's conjecture, namely, $|{\varphi }^{-1}(m)|\ge 2\forall m\in V$, and (ii) arithmetic and geometric progressions in $N_2$ and $N_3$. Finally, using the above sets associated to the totient function, we generate an infinite class of subsets of $\mathbb{N}$, each with asymptotic density zero and containing arbitrarily long arithmetic progressions.