# The 2-torsion subgroups of the class groups in families of cubic fields

NUMBER THEORY

##### Speaker:

Arul Shankar *- University of Toronto*

The Cohen--Lenstra--Martinet conjectures have been verified in

only two cases. Davenport--Heilbronn compute the average size of the

3-torsion subgroups in the class group of quadratic fields and Bhargava

computes the average size of the 2-torsion subgroups in the class groups of cubic fields. The values computed in the above two results are remarkably stable. In particular, work of Bhargava--Varma shows that they do not change if one instead averages over the family of quadratic or cubic fields satisfying any finite set of splitting conditions.

However for certain "thin" families of cubic fields, namely, families of

monogenic and n-monogenic cubic fields, the story is very different. In

this talk, we will determine the average size of the 2-torsion subgroups of

the class groups of fields in these thin families. Surprisingly, these

values differ from the Cohen--Lenstra--Martinet predictions! We will also

provide an explanation for this difference in terms of the Tamagawa numbers of naturally arising reductive groups. This is joint work with Manjul Bhargava and Jon Hanke.

Zoom: https://harvard.zoom.us/j/96767001802

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