On a problem related to “second” best approximations to a real number

Abstract

For a given irrational number $\alpha$ one can define an irrationality measure function $\psi_{\alpha}^{[2]}(t) = \min\limits_{\substack{(q, p)\colon q, p \in\mathbb{Z}, 1\leqslant q\leqslant t, \\ (p, q) \neq (p_n, q_n) ~\forall n\in\mathbb{Z_{+}}}} |q\alpha -p|$, related to the second-best approximations to $\alpha$. In 2017 Moshchevitin studied the corresponding Diophantine constant $\mathfrak{k}(\alpha) = \liminf\limits_{t\to\infty} t \cdot\psi_{\alpha}^{[2]}(t)$ and the corresponding spectrum $\mathbb{L}_2 = \{ \lambda ~| ~\exists\alpha\in\mathbb{R\setminus Q} \colon \lambda = \mathfrak{k}(\alpha) \}$. In particular, he calculated two largest elements of the spectrum $\mathbb{L}_2$. In the present paper we calculate the value for the third element of the spectrum $\mathbb{L}_2$.