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Congruences for odd class numbers of quadratic fields with odd discriminant

Number Theory 2022-01-31 v2

Abstract

For any distinct two primes p1p23p_1\equiv p_2\equiv 3 (mod 4)(\text{mod }4), let h(p1)h(-p_1), h(p2)h(-p_2) and h(p1p2)h(p_1p_2) be the class numbers of the quadratic fields Q(p1)\mathbb{Q}(\sqrt{-p_1}), Q(p2)\mathbb{Q}(\sqrt{-p_2}) and Q(p1p2)\mathbb{Q}(\sqrt{p_1p_2}), respectively. Let ωp1p2:=(1+p1p2)/2\omega_{p_1p_2}:=(1+\sqrt{p_1p_2})/2 and let Ψ(ωp1p2)\Psi(\omega_{p_1p_2}) be the Hirzebruch sum of ωp1p2\omega_{p_1p_2}. We show that h(p1)h(p2)h(p1p2)Ψ(ωp1p2)/nh(-p_1)h(-p_2)\equiv h(p_1p_2)\Psi(\omega_{p_1p_2})/n (mod 8)(\text{mod }8), where n=6n=6 (respectively, n=2n=2) if min{p1,p2}>3\min\{p_1,p_2\}>3 (respectively, otherwise). We also consider the real quadratic order with conductor 22 in Q(p1p2)\mathbb{Q}(\sqrt{p_1p_2}).

Keywords

Cite

@article{arxiv.2201.04291,
  title  = {Congruences for odd class numbers of quadratic fields with odd discriminant},
  author = {Jigu Kim and Yoshinori Mizuno},
  journal= {arXiv preprint arXiv:2201.04291},
  year   = {2022}
}

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19 pages