Rational isolated $j$-invariants from $X_1(\ell^n)$ and $X_0(\ell^n)$
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| Format: | Preprint |
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2025
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| _version_ | 1866912447026167808 |
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| author | Bourdon, Abbey Ejder, Özlem |
| author_facet | Bourdon, Abbey Ejder, Özlem |
| contents | Let $\ell$ and $n$ be positive integers with $\ell$ prime. The modular curves $X_1(\ell^n)$ and $X_0(\ell^n)$ are algebraic curves over $\mathbb{Q}$ whose non-cuspidal points parameterize elliptic curves with a distinguished point of order $\ell^n$ or a distinguished cyclic subgroup of order $\ell^n$, respectively. We wish to understand isolated points on these curves, which are roughly those not belonging to an infinite parameterized family of points having the same degree. Our first main result is that there are precisely 15 $j$-invariants in $\mathbb{Q}$ which arise as the image of an isolated point $x\in X_1(\ell^n)$ under the natural map $j:X_1(\ell^n) \rightarrow X_1(1)$. This completes a prior partial classification of Ejder. We also identify the 19 rational $j$-invariants which correspond to isolated points on $X_0(\ell^n)$. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2506_19560 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Rational isolated $j$-invariants from $X_1(\ell^n)$ and $X_0(\ell^n)$ Bourdon, Abbey Ejder, Özlem Number Theory 14G35, 11G05 Let $\ell$ and $n$ be positive integers with $\ell$ prime. The modular curves $X_1(\ell^n)$ and $X_0(\ell^n)$ are algebraic curves over $\mathbb{Q}$ whose non-cuspidal points parameterize elliptic curves with a distinguished point of order $\ell^n$ or a distinguished cyclic subgroup of order $\ell^n$, respectively. We wish to understand isolated points on these curves, which are roughly those not belonging to an infinite parameterized family of points having the same degree. Our first main result is that there are precisely 15 $j$-invariants in $\mathbb{Q}$ which arise as the image of an isolated point $x\in X_1(\ell^n)$ under the natural map $j:X_1(\ell^n) \rightarrow X_1(1)$. This completes a prior partial classification of Ejder. We also identify the 19 rational $j$-invariants which correspond to isolated points on $X_0(\ell^n)$. |
| title | Rational isolated $j$-invariants from $X_1(\ell^n)$ and $X_0(\ell^n)$ |
| topic | Number Theory 14G35, 11G05 |
| url | https://arxiv.org/abs/2506.19560 |