In Grassmannians, dimension 3 and codimension 3 classes are realizable by irreducible subvarieties exactly when b²≥ac, and in G(2,n) classes are realizable over Q exactly when their coefficients form a log-concave sequence with no internal zeros.
Multi-rigidity of Schubert classes in partial flag varieties
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abstract
In this paper, we study the multi-rigidity problem in rational homogeneous spaces. A Schubert class is called multi-rigid if every multiple of it can only be represented by a union of Schubert varieties. We prove the multi-rigidity of Schubert classes in rational homogeneous spaces. In particular, we characterize the multi-rigid Schubert classes in partial flag varieties of type A, B and D. Moreover, for a general rational homogeneous space $G/P$, we deduce the rigidity and multi-rigidity from the corresponding generalized Grassmannians (correspond to maximal parabolics). When $G$ is semi-simple, we also deduce the rigidity and multi-rigidity from the simple cases.
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Realization of Cohomology Classes in Grassmannians
In Grassmannians, dimension 3 and codimension 3 classes are realizable by irreducible subvarieties exactly when b²≥ac, and in G(2,n) classes are realizable over Q exactly when their coefficients form a log-concave sequence with no internal zeros.