Consider a proper, isometric action by a unimodular locally compact group G on a Riemannian manifold M with boundary, such that M/G is compact. Then an equivariant Dirac type operator on M under a suitable boundary condition has an equivariant index in the Ktheory of the reduced group C * -algebra C * r G of G, which is a common generalisation of the Baum-Connes analytic assembly map and the (equivariant) Atiyah-Patodi-Singer index. Using a trace on a suitable subalgebra of C * r G, defined by the orbital integral over the conjugacy class of an element g ∈ G, we prove an equivariant Atiyah-Patodi-Singer index theorem in this setting. We first state general analytic conditions under which this theorem holds, and then show that these conditions are satisfied if g = e is the identity element; if G is a finitely generated, discrete group, and the conjugacy class of g has polynomial growth; and if G is a connected, linear, real semisimple Lie group, and g is a semisimple element. In the classical case, where M is compact and G is trivial, our arguments reduce to a relatively short and simple proof of the original Atiyah-Patodi-Singer index theorem.
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