Augmented singular cohomology, uniform matroids, and real-rootedness

RESUMO

We study the singular cohomology rings of toric varieties associated with several fans arising from uniform matroids. These rings generalize the Chow and augmented Chow rings of matroids. For the singular cohomology ring arising from the augmented Bergman fan of a uniform matroid, we construct an explicit basis derived from the retral basis for the singular cohomology ring of a uniform matroid introduced by the first author. We prove that the augmented Bergman fan does not yield a singular cohomology ring that satisfies the quasi-projective Strong Lefschetz property, whereas a suitable modification of the fan does. We then investigate the zeros of the corresponding refined Hodge--Poincaré polynomials. For uniform matroids, we prove that the refined Hodge--Poincaré polynomials associated with both the singular cohomology ring and the modified augmented singular cohomology ring are real-rooted. The former result resolves a conjecture of the first author. These results extend the real-rootedness theorem of Brändén and the second author for the Chow polynomials of uniform matroids. Finally, we relate the failure of real-rootedness for the augmented singular cohomology ring to the failure of Lefschetz properties.