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HEURISTICS FOR $p$-CLASS TOWERS OF REAL QUADRATIC FIELDS

Part of: Algebraic number theory: global fields

Published online by Cambridge University Press:  26 December 2019

Nigel Boston ,
Michael R. Bush  and
Farshid Hajir
Nigel Boston
Affiliation:
Department of Mathematics, University of Wisconsin – Madison, 480 Lincoln Drive, Madison, WI 53706, USA ([email protected])
Michael R. Bush
Affiliation:
Department of Mathematics, Washington and Lee University, 204 W. Washington Street, Lexington, VA 24450, USA ([email protected])
Farshid Hajir
Affiliation:
Department of Mathematics & Statistics, University of Massachusetts – Amherst, 710 N. Pleasant Street, Amherst, MA 01003, USA ([email protected])
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Abstract

Let $p$ be an odd prime. For a number field $K$, we let $K_{\infty }$ be the maximal unramified pro-$p$ extension of $K$; we call the group $\text{Gal}(K_{\infty }/K)$ the $p$-class tower group of $K$. In a previous work, as a non-abelian generalization of the work of Cohen and Lenstra on ideal class groups, we studied how likely it is that a given finite $p$-group occurs as the $p$-class tower group of an imaginary quadratic field. Here we do the same for an arbitrary real quadratic field $K$ as base. As before, the action of $\text{Gal}(K/\mathbb{Q})$ on the $p$-class tower group of $K$ plays a crucial role; however, the presence of units of infinite order in the ground field significantly complicates the possibilities for the groups that can occur. We also sharpen our results in the imaginary quadratic field case by removing a certain hypothesis, using ideas of Boston and Wood. In the appendix, we show how the probabilities introduced for finite $p$-groups can be extended in a consistent way to the infinite pro-$p$ groups which can arise in both the real and imaginary quadratic settings.

Keywords

Cohen–Lenstra heuristicsclass field towerreal quadratic fieldideal class groupSchur 𝜎-groupSchur + 1 𝜎-group

MSC classification

Primary: 11R29: Class numbers, class groups, discriminants 11R11: Quadratic extensions
Secondary: 11R32: Galois theory
Type
Research Article
Information
Journal of the Institute of Mathematics of Jussieu , Volume 20 , Issue 4 , July 2021 , pp. 1429 - 1452
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Copyright
© The Author(s), 2019. Published by Cambridge University Press

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Footnotes

The research of NB is supported by Simons grant MSN179747. The work of MRB was partially supported by summer Lenfest Grants from Washington and Lee University.

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