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 $c_{ L/k}^*$.
Categories: math.KT math.AT math.RA