A representation theoretic classification of multiprojective spaces

Abstract

For a partition $(n_1,\ldots,n_r)$ of a positive integer $n$, consider the associated multiprojective space $\mathbb{P}^{n_1}\times\cdots\times\mathbb{P}^{n_r}$. That multiprojective spaces attached to distinct partitions of $n$ are pairwise non-isomorphic is known, having been established by algebro-geometric methods. In this paper we give a new, representation-theoretic proof. We prove that the complex cohomology ring of a multiprojective space carries a natural $\mathfrak{sl}(2,\mathbb{C})$-module structure compatible with the Künneth decomposition. The classification then follows from C. S. Rajan's theorem on the unique decomposition of tensor products of irreducible representations of a simple Lie algebra. We further show, by a dimension count argument, that this approach is intrinsic to the genus zero curve, distinguishing $\mathbb{P}^1$ from curves of higher genus.