Hilbert scheme of rational curves on a generic quintic threefold

Abstract

Let $X_0$ be a generic quintic threefold in projective space ${\mathbf{P}}^4$ over the complex numbers. For a fixed natural number $d$, let $R_d(X_0)$ be the open sub-scheme of the Hilbert scheme, parameterizing irreducible rational curves of degree $d$ on $X_0$. In this paper, we show that (1) $R_d(X_0)$ is smooth and of expected dimension, \par (2) Combining the Calabi-Yau condition on $X_0$, we further show that it consists of immersed rational curves. (3) Parts (1) and (2) imply a statement of Clemens' conjecture: if $C_0\in R_d(X_0)$ and $c_0:{\mathbf{P}}^1\to C_0$ is the normalization, the \par\hspace{1cc} normal sheaf is isomorphic to the vector bundle $$N_{c_0/X_0}\simeq {\mathcal{O}}_{{\mathbf{P}}^1}(-1)\oplus {\mathcal{O}}_{{\mathbf{P}}^1}(-1).$$