Classification of finite W-groups

Abstract

We determine the structure of the W-group $\mathcal{G}_F$, the small Galois quotient of the absolute Galois group $G_F$ of the Pythagorean formally real field $F$ when the space of orderings $X_F$ has finite order. Based on Marshall's work (1979), we reduce the structure of $\mathcal{G}_F$ to that of $\mathcal{G}_{\bar{F}}$, the W-group of the residue field $\bar{F}$ when $X_F$ is a connected space. In the disconnected case, the structure of $\mathcal{G}_F$ is the free product of the W-groups $\mathcal{G}_{F_i}$ corresponding to the connected components $X_i$ of $X_F$. We also give a completely Galois theoretic proof for Marshall's Basic Lemma.