Boundaries, spectral triples and K-homology
Journal article, 2019

© European Mathematical Society This paper extends the notion of a spectral triple to a relative spectral triple, an unbounded analogue of a relative Fredholm module for an ideal J G A. Examples include manifolds with boundary, manifolds with conical singularities, dimension drop algebras, -deformations and Cuntz–Pimsner algebras of vector bundles. The bounded transform of a relative spectral triple is a relative Fredholm module, making the image of a relative spectral triple under the boundary mapping in K-homology easy to compute. We introduce an additional operator called a Clifford normal with which a relative spectral triple can be doubled into a spectral triple. The Clifford normal also provides a boundary Hilbert space, a representation of the quotient algebra, a boundary Dirac operator and an analogue of the Calderon projection. In the examples this data does assemble to give a boundary spectral triple, though we can not prove this in general. When we do obtain a boundary spectral triple, we provide sufficient conditions for the boundary triple to represent the K-homological boundary. Thus we abstract the proof of Baum–Douglas–Taylor’s “boundary of Dirac is Dirac on the boundary” theorem into the realm of non-commutative geometry.

Spectral triple

K-homology

Manifold-with-boundary

Author

Iain Forsyth

University of Hanover

Magnus C H T Goffeng

Chalmers, Mathematical Sciences, Analysis and Probability Theory

Bram Mesland

University of Hanover

Adam Rennie

University of Wollongong

Journal of Noncommutative Geometry

1661-6952 (ISSN) 1661-6960 (eISSN)

Vol. 13 2 407-472

Subject Categories

Algebra and Logic

Geometry

Mathematical Analysis

DOI

10.4171/JNCG/331

More information

Latest update

9/20/2019