Injectivity and Surjectivity of the Dress Map

Abstract

For a nontrivial finite Galois extension $L/k$ (where the characteristic of $k$ is different from 2) with Galois group $G$, we prove that the Dress map $h_{L/k}: A(G) \to GW(k)$ is injective if and only if $L=k(\sqrt{\alpha})$ where $\alpha$ is not a sum of squares in $k^\times$. Furthermore, we prove that $h_{L/k}$ is surjective if and only if $k$ is quadratically closed in $L$. As a consequence, we give strong necessary conditions for faithfulness of the Heller-Ormsby functor $c_{L/k}^* : \text{SH}_G\to \text{SH}_k$, as well as strong necessary conditions for fullness of $c_{L/k}^*$.