Rational Isolated j-invariants from X1(ℓn) and X0(ℓn)

Abstract

Let ℓ and n be positive integers with ℓ prime. The modular curves X1(ℓn) and X0(ℓn) are algebraic curves over Q whose non-cuspidal points parameterize elliptic curves with a distinguished point of order ℓn or a distinguished cyclic subgroup of order ℓn, 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 Q which arise as the image of an isolated point x∈X1(ℓn) under the natural map j:X1(ℓn)→X1(1). This completes a prior partial classification of Ejder. We also identify the 19 rational j-invariants which correspond to isolated points on X0(ℓn). © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2026.