To a maximal torus in a quasi-split semi-simple simply-connected group over a
local field of characteristic 0, Langlands and Shelstad construct a
cohomological invariant called the splitting invariant, which is an important
component of their endoscopic transfer factors. We study this invariant in the
case of a split real group and prove a decomposition theorem which expresses
this invariant for a general torus as a product of the corresponding invariants
for simple tori. We also show how this reduction formula allows for the
comparison of splitting invariants between different tori in the given real
group.Comment: 22 page