Realizability of fusion systems by discrete groups

For a prime p p , fusion systems over discrete p p -toral groups are categories that model and generalize the p p -local structure of Lie groups and certain other infinite groups in the same way that fusion systems over finite p p -groups model and generalize the p p -local structure of finite groups. In the finite case, it is natural to say that a fusion system F \mathcal {F} is realizable if it is isomorphic to the fusion system of a finite group, but it is less clear what realizability should mean in the discrete p p -toral case. In this paper, we look at some of the different types of realizability for fusion systems over discrete p p -toral groups, including realizability by linear torsion groups and sequential realizability, of which the latter is the most general. After showing that fusion systems of compact Lie groups are always realized by linear torsion groups (hence sequentially realizable), we give some new tools for showing that certain fusion systems are not sequentially realizable, and illustrate it with two large families of examples.