On the geometry of the second Lagrange spectra
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arXiv
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| Format: | Preprint |
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2026
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| _version_ | 1866908825088425984 |
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| author | Cheng, Hao Erazo, Harold Moreira, Carlos Gustavo Vasconcelos, Thiago |
| author_facet | Cheng, Hao Erazo, Harold Moreira, Carlos Gustavo Vasconcelos, Thiago |
| contents | The Lagrange spectrum $L$ is the set of finite values of the best approximation constants $k(α)=\limsup_{|p|,|q|\to \infty}|q(qα-p)|^{-1}$, where $α\in \mathbb{R}\setminus \mathbb{Q}$. It is a classical result that the pairs $(p,q)$ attaining these approximation constants arise from the convergents $(p_n,q_n)$ of the continued fraction of $α$. Consequently, $k(α)=\limsup_{n\to\infty}|q_n(q_nα-p_n)|^{-1}$. Moreira proved that the function $d(t)=HD(L\cap(-\infty,t))$ where $HD$ denotes Hausdorff dimension, is continuous.
Second Lagrange spectra are defined analogously to the classical Lagrange spectrum, but are associated with the problem of approximating an irrational number $α$ by rational numbers $\frac{p}{q}$ that are not convergents of its continued fraction expansion. Two natural definitions arise depending on whether rational multiples $(p,q)=(kp_n,kq_n),k\geq 2$ which represent the same rational numbers as convergents, are allowed or excluded. Based on this distinction, Moshchevitin introduced two second Lagrange spectra, denoted $L_2$ and $L_2^*$.
We prove that the function $d_2(t)=HD(L_2\cap (-\infty,t))$ is continuous, whereas $d_2^*(t)=HD(L_2^*\cap (-\infty,t))$ is discontinuous and assumes only the values 0 and 1. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_09228 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | On the geometry of the second Lagrange spectra Cheng, Hao Erazo, Harold Moreira, Carlos Gustavo Vasconcelos, Thiago Number Theory Dynamical Systems 11J70, 11J70, 28A78, 37D05 The Lagrange spectrum $L$ is the set of finite values of the best approximation constants $k(α)=\limsup_{|p|,|q|\to \infty}|q(qα-p)|^{-1}$, where $α\in \mathbb{R}\setminus \mathbb{Q}$. It is a classical result that the pairs $(p,q)$ attaining these approximation constants arise from the convergents $(p_n,q_n)$ of the continued fraction of $α$. Consequently, $k(α)=\limsup_{n\to\infty}|q_n(q_nα-p_n)|^{-1}$. Moreira proved that the function $d(t)=HD(L\cap(-\infty,t))$ where $HD$ denotes Hausdorff dimension, is continuous. Second Lagrange spectra are defined analogously to the classical Lagrange spectrum, but are associated with the problem of approximating an irrational number $α$ by rational numbers $\frac{p}{q}$ that are not convergents of its continued fraction expansion. Two natural definitions arise depending on whether rational multiples $(p,q)=(kp_n,kq_n),k\geq 2$ which represent the same rational numbers as convergents, are allowed or excluded. Based on this distinction, Moshchevitin introduced two second Lagrange spectra, denoted $L_2$ and $L_2^*$. We prove that the function $d_2(t)=HD(L_2\cap (-\infty,t))$ is continuous, whereas $d_2^*(t)=HD(L_2^*\cap (-\infty,t))$ is discontinuous and assumes only the values 0 and 1. |
| title | On the geometry of the second Lagrange spectra |
| topic | Number Theory Dynamical Systems 11J70, 11J70, 28A78, 37D05 |
| url | https://arxiv.org/abs/2602.09228 |