Bost-Connes type systems for function fields

Abstract

We describe a construction which associates to any function field $k$ and any place $\infty$ of $k$ a $C^*$-dynamical system $(C_{k,\infty},\sigma_t)$ that is analogous to the Bost-Connes system associated to $\QQ$ and its archimedian place. Our construction relies on Hayes' explicit class field theory in terms of sign-normalized rank one Drinfel'd modules. We show that $C_{k,\infty}$ has a faithful continuous action of $\Gal(K/k)$, where $K$ is a certain field constructed by Hayes, such that $k^\abi\subset K\subset k^\ab$, where $k^\abi$ is the maximal abelian extension of $k$ that is totally split at $\infty$. We classify the extremal KMS$_\beta$ states of $(C_{k,\infty},\sigma_t)$ at any temperature $0<1/\beta<\infty$ and show that a phase transition with spontaneous symmetry breaking occurs at temperature $1/\beta=1$. At high temperature $1/\beta\geqslant 1$, there is a unique KMS$_\beta$ state. At low temperature $1/\beta<1$, the space of extremal KMS$_\beta$ states is principal homogeneous under $\Gal(K/k)$. Each such state is of type $\I_\infty$ and the partition function is the Dedekind zeta function $\zeta_{k,\infty}$. Moreover, we construct a "rational" *-subalgebra $\HH$, we give a presentation of $\HH$ and of $C_{k,\infty}$, and we show that the values of the low-temperature extremal KMS$_\beta$ states at certain elements of $\HH$ are related to special values of partial zeta functions.\n Erratum: This article wrongly claims that at high temperature $1/\beta\geqslant 1$, the unique KMS$_\beta$ state is of type $\III_{q^{-\beta}}$, where $q$ is the cardinal of the constant subfield of $k$. It has been shown by Neshveyev and Rustad \cite{NesRus12} that the correct type is $\III_{q^{-\beta d_\infty}}$ where $d_\infty$ is the degree of the place $\infty$. The original statements have been kept for reference, but errata have been inserted next to them.