# Bost–Connes systems, Hecke algebras, and induction

### Marcelo Laca

University of Victoria, Canada### Sergey Neshveyev

University of Oslo, Norway### Mak Trifković

University of Victoria, Canada

## Abstract

We consider a Hecke algebra naturally associated with the affine group with totally positive multiplicative part over an algebraic number field $K$ and we show that the C*-algebra of the Bost–Connes system for $K$ can be obtained from our Hecke algebra by induction, from the group of totally positive principal ideals to the whole group of ideals. Our Hecke algebra is therefore a full corner, corresponding to the narrow Hilbert class field, in the Bost–Connes C*-algebra of $K$; in particular, the two algebras coincide if and only if $K$ has narrow class number one. Passing the known results for the Bost–Connes system for $K$ to this corner, we obtain a phase transition theorem for our Hecke algebra.

In another application of induction we consider an extension $L/K$ of number fields and we show that the Bost–Connes system for $L$ embeds into the system obtained from the Bost–Connes system for $K$ by induction from the group of ideals in $K$ to the group of ideals in $L$. This gives a C*-algebraic correspondence from the Bost–Connes system for $K$ to that for $L$. Therefore the construction of Bost–Connes systems can be extended to a functor from number fields to C*-dynamical systems with equivariant correspondences as morphisms. We use this correspondence to induce KMS-states and we show that for $β>1$ certain extremal KMS$_{β}$-states for $L$ can be obtained, via induction and rescaling, from KMS$_{[L:K]β}$-states for $K$. On the other hand, for $0<β≤1$ every KMS$_{[L:K]β}$-state for $K$ induces to an infinite weight.

## Cite this article

Marcelo Laca, Sergey Neshveyev, Mak Trifković, Bost–Connes systems, Hecke algebras, and induction. J. Noncommut. Geom. 7 (2013), no. 2, pp. 525–546

DOI 10.4171/JNCG/125