English

Parameterizing roots of polynomial congruences

Number Theory 2022-08-17 v2

Abstract

We use the arithmetic of ideals in orders to parameterize the roots μ(modm)\mu \pmod m of the polynomial congruence F(μ)0(modm)F(\mu) \equiv 0 \pmod m, F(X)Z[X]F(X) \in \mathbb{Z}[X] monic, irreducible and degree dd. Our parameterization generalizes Gauss's classic parameterization of the roots of quadratic congruences using binary quadratic forms, which had previously only been extended to the cubic polynomial F(X)=X32F(X) = X^3 - 2. We show that only a special class of ideals are needed to parameterize the roots μ(modm)\mu \pmod m, and that in the cubic setting, d=3d = 3, general ideals correspond to pairs of roots μ1(modm1)\mu_1 \pmod{m_1}, μ2(modm2)\mu_2 \pmod {m_2} satisfying gcd(m1,m2,μ1μ2)=1\gcd(m_1, m_2, \mu_1 - \mu_2) = 1. At the end we illustrate our parameterization and this correspondence between roots and ideals with a few applications, including finding approximations to μmR/Z\frac{\mu}{m} \in \mathbb{R}/ \mathbb{Z}, finding an explicit Euler product for the co-type zeta function of Z[213]\mathbb{Z}[2^{\frac{1}{3}}], and computing the composition of cubic ideals in terms of the roots μ1(modm1)\mu_1 \pmod {m_1} and μ2(modm2)\mu_2 \pmod {m_2}.

Keywords

Cite

@article{arxiv.2008.00538,
  title  = {Parameterizing roots of polynomial congruences},
  author = {Matthew Welsh},
  journal= {arXiv preprint arXiv:2008.00538},
  year   = {2022}
}

Comments

52 pages, to be published in Algebra and Number Theory

R2 v1 2026-06-23T17:35:14.699Z