Transfer and characteristic idempotents for saturated fusion systems

We construct a well-behaved transfer map from the p-local Burnside ring of the underlying p-group S to the p  -local Burnside ring of a saturated fusion system FF. Using this transfer map, we give new results on the characteristic idempotent of FF – the unique idempotent in the p-local double Burnside ring of S satisfying properties of Linckelmann and Webb. We describe this idempotent explicitly both in terms of fixed points and as a linear combination of transitive bisets. Additionally, using fixed points we determine the map for Burnside rings given by multiplication with the characteristic idempotent, and show that this is the transfer map previously constructed. Applying these results, we show that for every saturated fusion system the ring generated by all (not necessarily idempotent) characteristic elements in the p-local double Burnside ring is isomorphic as rings to the p-local “single” Burnside ring of the fusion system, and we disprove a conjecture by Park–Ragnarsson–Stancu on the composition product of fusion systems.