Application of semifinite index theory to weak topological phases

Recent work by Prodan and the second author showed that weak invariants of topological insulators can be described using Kasparov’s $KK$-theory. In this note, a complementary description using semifinite index theory is given. This provides an alternative proof of the index formulae for weak complex topological phases using the semifinite local index formula. Real invariants and the bulk-boundary correspondence are also briefly considered.