Filtrations of Tope Spaces of Oriented Matroids

TL;DR

This paper compares three filtrations of the tope space of an oriented matroid, showing their equivalence over certain coefficients and exploring their applications in real algebraic geometry.

Contribution

It demonstrates the coincidence of three filtrations over 0 coefficients and extends the Varchenko-Gelfand filtration to a 0 integral setting with applications.

Findings

All three filtrations coincide over 0 coefficients.

The dual Varchenko-Gelfand filtration can be extended to a 0-sign cosheaf.

Applications to real algebraic geometry via patchworking.

Abstract

We compare three filtrations of the tope space of an oriented matroid. The first is the dual Varchenko-Gelfand degree filtration, the second filtration is from Kalinin's spectral sequence, and the last one derives from Quillen's augmentation filtration. We show that all three filtrations and the respective maps coincide over $\mathbb{Z}/ 2\mathbb{Z}$. We also show that the dual Varchenko-Gelfand degree filtration can be made into a filtration of the $\mathbb{Z}$-sign cosheaf on the fan of the underlying matroid. This was previously carried out with $\mathbb{Z}/ 2\mathbb{Z}$-coefficients by the first author and Renaudineau using the Quillen filtration and has applications to real algebraic geometry via patchworking.