# Variations on Poincar\'e duality for intersection homology

## TL;DR

This paper explores various forms of Poincaré duality in intersection homology with ring coefficients, providing explicit examples, duality constructions, and conditions for duality to hold in singular spaces.

## Contribution

It introduces a duality framework from cap products, compares two intersection cohomologies, and identifies the main obstruction to Poincaré duality, including explicit computations and conditions involving torsion.

## Key findings

- Poincaré duality can hold with torsion in the critical degree.
- A duality from cap products links two intersection cohomologies.
- The main obstruction is the homology of a specific complex.

## Abstract

Intersection homology with coefficients in a field restores Poincar\'e duality for some spaces with singularities, as pseudomanifolds. But, with coefficients in a ring, the behaviours of manifolds and pseudomanifolds are different. This work is an overview, with proofs and explicit examples, of various possible situations with their properties. We first set up a duality, defined from a cap product, between two intersection cohomologies: the first one arises from a linear dual and the second one from a simplicial blow up. Moreover, from this property, Poincar\'e duality in intersection homology looks like the Poincar\'e-Lefschetz duality of a manifold with boundary. Besides that, an investigation of the coincidence of the two previous cohomologies reveals that the only obstruction to the existence of a Poincar\'e duality is the homology of a well defined complex. This recovers the case of the peripheral sheaf introduced by Goresky and Siegel for compact PL-pseudomanifolds. We also list a series of explicit computations of peripheral intersection cohomology. In particular, we observe that Poincar\'e duality can exist in the presence of torsion in the "critical degree" of the intersection homology of the links of a pseudomanifold.