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Sporadic points of odd degree on $X_1(N)$ coming from $\mathbb{Q}$-curves

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arxiv 2107.10909 v2 pith:PXEIMHNP submitted 2021-07-22 math.NT

classification math.NT
keywords curvesellipticmathbbsporadiccurvepointsdegreeclass
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abstract

We say a closed point $x$ on a curve $C$ is sporadic if there are only finitely many points on $C$ of degree at most deg$(x)$. In the case where $C$ is the modular curve $X_1(N)$, most known examples of sporadic points come from elliptic curves with complex multiplication (CM). We seek to understand all sporadic points on $X_1(N)$ corresponding to $\mathbb{Q}$-curves, which are elliptic curves isogenous to their Galois conjugates. This class contains not only all CM elliptic curves, but also any elliptic curve $\overline{\mathbb{Q}}$-isogenous to one with a rational $j$-invariant, among others. In this paper, we show that all non-CM $\mathbb{Q}$-curves giving rise to a sporadic point of odd degree lie in the $\overline{\mathbb{Q}}$-isogeny class of the elliptic curve with $j$-invariant $-140625/8$. In addition, we show that a stronger version of this finiteness result would imply Serre's Uniformity Conjecture.

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Cited by 3 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Isogeny graphs of elliptic curves in characteristic zero

    math.NT 2026-08 conditional novelty 8.0 of 10

    Isogeny graphs of non-CM elliptic curves in characteristic 0 decompose into p-primary pieces, each isomorphic to one of the explicit trees H^r_{p^k}, H^r_{p∞}, or H^r_{p∞,+}, with r governed by a new p-blooming invariant.

  2. Rational isolated $j$-invariants from $X_1(\ell^n)$ and $X_0(\ell^n)$

    math.NT 2025-06 accept novelty 7.0 of 10

    For prime-power level, rational isolated j-invariants are exactly 15 values on X_1 (13 CM plus two non-CM) and 19 values on X_0 (13 CM plus six non-CM).

  3. Computing the Level of a Fiber for Points on Modular Curves

    math.NT 2025-08 unverdicted novelty 6.0 of 10

    A new 'level of a fiber' invariant is defined for points on X1(l^n), and a Lang-Trotter-style theorem shows that maximal degree at level k+1 forces maximal degree for all higher lifts.

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