The Noncommutative Foliation Invariant (NCFI): extension to the the odd codimension case and computed examples

This article extends the definition of the Noncommutative Foliation Invariant (NCFI) for foliations of odd codimension and computes certain key examples for both even and odd codimension cases: fibrational foliations, irrational Kronecker, rational Kronecker (both the vertical and the horzontal foliation using a flat connection) and weighted Hopf/orbifold cases.

We also prove some more general results along the way.

More concretely, we compute the Noncommutative Foliation Invariant (NCFI) for several basic families of foliated manifolds.

In even codimension the invariant is the Chern--Connes pairing of Connes' transverse fundamental cyclic cocycle with the \(K_0\)-class associated with the transverse geometric module.

In odd codimension the transverse cocycle has odd degree, so a numerical pairing also requires a specified odd \(K_1\)-class; the odd-codimensional value is therefore an invariant of the foliation together with this chosen odd-favourable structure.