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arXiv · 2412.11759

The external activity complex of a pair of matroids

Andrew Berget·Alex Fink
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Abstract

We introduce the Schubert variety of a pair of linear subspaces in $\mathbf{C}^n$ and the external activity complex of a pair of not necessarily realizable matroids. Both of these generalize constructions of Ardila et al., which occur when one of the linear spaces is one-dimensional. We prove that our external activity complex is Cohen-Macaulay and deduce a formula for its $K$-polynomial in terms of exterior powers of the dual tautological quotient classes of matroids. As a consequence, we deduce a non-negative formula for the matroid invariant $\omega(M)$ of Fink, Shaw, and Speyer in terms of certain homology groups of links within an external activity complex, proving the 2005 tropical $f$-vector conjecture of Speyer.

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Andrew Berget, Alex Fink. 2024-12-16. The external activity complex of a pair of matroids. https://arxiv.org/abs/2412.11759

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