On the tangent bundle and the divisor theory of a general matroid

Abstract

Extending classical algebro-geometric constructions to arbitrary matroids, we construct a $K$-class $T_M\in K(M)$ for every loopless matroid $M$. When $M$ is realizable by a linear subspace $L$, $T_M$ recovers the $K$-class of the tangent bundle of the wonderful compactification $W_L$. We derive two formulas for the total Chern class of $T_M$ (one combinatorial and one geometric) and show that the associated Todd class agrees with the Todd class appearing in the matroid Hirzebruch--Riemann--Roch formula. To develop a positivity theory entirely at the combinatorial level, we introduce the notion of ``fake effective cone,'' a combinatorial analogue of the classical effective cone, and use it to characterize big and nef divisors in $A(M)$. Finally, we define the ${\beta }_S$ classes, obtained from Cremona conjugates of the classical ${\alpha }_S$ classes, and study their properties to provide a rich and computable family of combinatorially nef divisors.

31 pages