Tangent classes for matroid building sets

Abstract

Let \(M\) be a loopless matroid on a finite ground set \(E\), and let \(\G\) be a building set containing the top flat \(E\). We define a tangent class \(T_{M,\G}\) in the \(K\)-ring \(K(M,\G)\), which extends the tangent bundle class of the de Concini--Procesi wonderful model from realizable matroids to arbitrary matroids with building sets. The class \(T_{M,\G}\) satisfies a matroidal Hirzebruch--Riemann--Roch package. More precisely, its Hirzebruch class \[ \operatorname{ch}(λ_y T_{M,\G}^{\vee})\operatorname{td}(T_{M,\G}) \] specializes to the Todd class and computes the Chow polynomial of \((M,\G)\). In the realizable case, these identities agree with the usual tangent-bundle computations on the corresponding wonderful model. As an application, we prove Chern-number inequalities for \(T_{M,\G}\), including a Miyaoka--Yau type inequality with respect to the hyperplane class.