Representation ring of Levi subgroups versus cohomology ring of flag varieties III

Abstract

For any reductive group $G$ and a parabolic subgroup $P$ with its Levi subgroup $L$, the first author [Ku2] introduced a ring homomorphism $ \xi^P_\lambda: Rep^\mathbb{C}_{\lambda-poly}(L) \to H^*(G/P, \mathbb{C})$, where $ Rep^\mathbb{C}_{\lambda-poly}(L)$ is a certain subring of the complexified representation ring of $L$ (depending upon the choice of an irreducible representation $V(\lambda)$ of $G$ with highest weight $\lambda$). In this paper we study this homomorphism for $G=SO(2n)$ and its maximal parabolic subgroups $P_{n-k}$ for any $2\leq k\leq n-1$ (with the choice of $V(\lambda) $ to be the defining representation $V(\omega_1) $ in $\mathbb{C}^{2n}$). Thus, we obtain a $\mathbb{C}$-algebra homomorphism $ \xi_{n,k}^D: Rep^\mathbb{C}_{\omega_1-poly}(SO(2k)) \to H^*(OG(n-k, 2n), \mathbb{C})$. We determine this homomorphism explicitly in the paper. We further analyze the behavior of $ \xi_{n,k}^D$ when $n$ tends to $\infty$ keeping $k$ fixed and show that $ \xi_{n,k}$ becomes injective in the limit. We also determine explicitly (via some computer calculation) the homomorphism $ \xi^P_\lambda$ for all the exceptional groups $G$ (with a specific `minimal' choice of $\lambda$) and all their maximal parabolic subgroups except $E_8$.