Relative $B$-groups

Abstract

This paper extends the notion of $B$-group to a relative context. For a finite group $K$ and a field $\mathbb{F}$ of characteristic 0, the lattice of ideals of the Green biset functor $\mathbb{F}B_K$ obtained by shifting the Burnside functor $\mathbb{F}B$ by $K$ is described in terms of {\em $B_K$-groups}. It is shown that any finite group $(L,\varphi)$ over $K$ admits a {\em largest quotient $B_K$-group} $\beta_K(L,\varphi)$. The simple subquotients of $\mathbb{F}B_K$ are parametrized by $B_K$-groups, and their evaluations can be precisely determined. Finally, when $p$ is a prime, the restriction $\mathbb{F}B_K^{(p)}$ of $\mathbb{F}B_K$ to finite $p$-groups is considered, and the structure of the lattice of ideals of the Green functor $\mathbb{F}B_K^{(p)}$ is described in full detail. In particular, it is shown that this lattice is always finite.