Rational curves in a quadric threefold via an $\text{SL}(2,\mathbb{C})$-representation

Abstract

In this paper, we regard the smooth quadric threefold $Q_3$ as Lagrangian Grassmannian and search for fixed rational curves of low degree in $Q_3$ with respect to a torus action, which is the maximal subgroup of the special linear group $\text{SL}(2,\mathbb{C})$. Most of them are confirmations of very well-known facts. If the degree of a rational curve is $3$, it is confirmed using the Lagrangian's geometric properties that the moduli space of twisted cubic curves in $Q_3$ has a specific projective bundle structure. From this, we can immediately obtain the cohomology ring of the moduli space.

20 pages, Revised, Published version in Journal of the Korean Mathematical Society (2025)