Equivariant linear isometries operads over Abelian groups
$N_\infty$-operads are an equivariant generalization of $E_\infty$-operads introduced by Blumberg and Hill to study structural problems in equivariant stable homotopy theory. In the original paper introducing these objects, Blumberg and Hill raised the question of classifying $N_\infty$-operads that are weakly equivalent to a particularly nice kind of $N_\infty$-operad called a linear isometries operad. For some groups there is a known classification of linear isometries operads up to weak equivalence in terms of certain combinatorially defined objects called saturated transfer systems, but this classification is known to be invalid in general. Various authors have made incremental progress on understanding the domain of validity for this classification, but even among cyclic groups the validity is unknown in general. We determine essentially all the finite Abelian groups for which the classification is valid using techniques from algebra and extremal combinatorics.