Erdős's eventual-time conjecture for quadratic character sums

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Let obreak\footcomment obreak\footcomment be omitted. For an odd \prime pp, write

Sℓ(p):=∑n≤ℓ(np),S_\ell(p):=\sum_{n\leq \ell}\left(\frac{n}{p}\right),

where (⋅p)\left(\frac{\cdot}{p}\right) is the Legendre symbol modulo pp. For ε>0\varepsilon>0, let Fε(p)F_\varepsilon(p) be the smallest integer such that

Sℓ(p)<εℓfor every ℓ≥Fε(p).S_\ell(p)<\varepsilon\ell\qquad\text{for every }\ell\geq F_\varepsilon(p).

Erdős's eventual-time conjecture. There exists a constant Cε>0C_\varepsilon>0 such that

∑p≤xFε(p)∼Cε xlog⁡x.\sum_{p\leq x}F_\varepsilon(p)\sim C_\varepsilon\,\frac{x}{\log x}.

Erdős posed this as a question about the eventual time at which a quadratic character sum remains below a linear barrier. The conjecture was proved by Elliott.

References

Primary source

Quanyu Tang and Hao Zhang, “Average first-passage times for character sums”, arXiv:2512.24631 (2026).

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