  Arthur's conjectures predict the existence of some very interesting unitary
representations occurring in spaces of automorphic forms. We prove the
unitarity of the "Langlands element" (i.e., the one specified by Arthur) of all
unipotent Arthur packets for split real groups. The proof uses Eisenstein
series, Langlands' constant term formula and square integrability criterion,
analytic properties of intertwining operators, and some mild arithmetic input
from the theory of Dirichlet L-functions, to reduce to a more combinatorial
problem about intertwining operators.
