# Analogies between group actions on 3-manifolds and number fields

### Adam S. Sikora

University at Buffalo SUNY, USA

## Abstract

Let a cyclic group $G$ act either on a number field $\mathbb L$ or on a $3$-manifold $M$. Let $s_{\mathbb L}$ be the number of ramified primes in the extension $\mathbb L^G\subset \mathbb L$ and $s_M$ be the number of components of the branching set of the branched covering $M\to M/G$. In this paper, we prove several formulas relating $s_{\mathbb L}$ and $s_M$ to the induced $G$-action on $Cl(\mathbb L)$ and $H_1(M),$ respectively. We observe that the formulas for $3$-manifolds and number fields are almost identical, and therefore, they provide new evidence for the correspondence between $3$-manifolds and number fields postulated in arithmetic topology.

## Cite this article

Adam S. Sikora, Analogies between group actions on 3-manifolds and number fields. Comment. Math. Helv. 78 (2003), no. 4, pp. 832–844

DOI 10.1007/S00014-003-0781-X