Transversals, duality, and irrational rotation

Abstract

An early result of Noncommutative Geometry was Connes' observation in the 1980's that the Dirac-Dolbeault cycle for the $2$-torus $\mathbb{T}^2$, which induces a Poincar\'e self-duality for $\mathbb{T}^2$, can be 'quantized' to give a spectral triple and a K-homology class in $KK_0(A_\theta\otimes A_\theta, \mathbb{C})$ providing the co-unit for a Poincar\'e self-duality for the irrational rotation algebra $A_\theta$ for any $\theta\in \mathbb{R}\setminus \mathbb{Q}$. This spectral triple has been extensively studied since. Connes' proof, however, relied on a K-theory computation and does not supply a representative cycle for the unit of this duality. Since such representatives are vital in applications of duality, we supply such a cycle in unbounded form in this article. Our approach is to construct, for any non-trivial element $g$ of the modular group, a finitely generated projective module $\mathcal{L}_g$ over $A_\theta \otimes A_\theta$ by using a reduction-to-a-transversal argument of Muhly, Renault, and Williams, applied to a pair of Kronecker foliations along lines of slope $\theta$ and $g(\theta)$, using the fact that these flows are transverse to each other. We then compute Connes' dual of $[\mathcal{L}_g]$ for $g$ upper triangular, and prove that we obtain an invertible in $KK_0(A_\theta, A_\theta)$, represented by what one might regard as a noncommutative bundle of Dirac-Schr\"odinger operators. An application of $\mathbb{Z}$-equivariant Bott Periodicity proves that twisting the module by the family gives the requisite spectral cycle for the unit, thus proving self-duality for $A_\theta$ with both unit and co-unit represented by spectral cycles.