diff --git a/01-intro.tex b/01-intro.tex index 942f85e..49cfdc0 100644 --- a/01-intro.tex +++ b/01-intro.tex @@ -8,16 +8,45 @@ \section{The $\cC$-Albanese morphism in the presence of rational curves} \subversionInfo -\begin{thm}\label{thm:1} +\begin{thm}\label{thm:1}% Let $X$ be a projective manifold and let $x \in X$ be any point. If $C \subset - X$ is a rational curve, then the Albanese morphism of the $\cC$-pair $(X,0)$, + X$ is a rational curve, then the Albanese morphism\watchOut{Stefan 03Jun24: + Need to make assumptions to ensure that the Albanese exists.} of the + $\cC$-pair $(X,0)$, \[ \alb_x(X,0) : X \to \Alb_x(X,0), \] maps the curve $C$ to a point. \end{thm} \begin{proof} - \todo{PENDING} + The normalization of $C$ yields a diagram + \[ + \begin{tikzcd}[column sep=2cm] + \bP¹ \ar[r, "n\text{, normalization}"'] \ar[rr, bend left=15, "\eta"] & C \ar[r, "\text{inclusion}"'] & X. + \end{tikzcd} + \] + Consider the point $y := n(0_{\bP¹}) \in X$. It follows from the universal + property of the Albanese that the Albanese varieties $\Alb_x(X,0)$ and + $\Alb_y(X,0)$ are isomorphic. To be more precise, there exists a unique Lie + group isomorphism $t$ that makes the following diagram commute, + \[ + \begin{tikzcd}[column sep=2cm] + X \ar[r, "\alb_x(X{,}0)"] \ar[d, equal] & \Alb_x(X,0) \ar[d, two heads, hook, "t"] \\ + X \ar[r, "\alb_y(X{,}0)"'] & \Alb_y(X,0). + \end{tikzcd} + \] + We may therefore assume without loss of generality that $x = y = n(0_{\bP¹})$. + + Given that $X$ is smooth, recall from \cite{orbiAlb1} that $\eta$ is a + $\cC$-morphism, between $\cC$-pairs $(\bP¹, 0)$ and $(X, 0)$. Invoking the + universal property of the Albanese once more, we find an analogous diagram + \[ + \begin{tikzcd}[column sep=2cm] + \bP¹ \ar[r, "\alb_0(\bP¹{,}0)"] \ar[d, equal] & \Alb_0(\bP¹,0) \ar[d] \\ + X \ar[r, "\alb_x(X{,}0)"'] & \Alb_x(X,0). + \end{tikzcd} + \] + The claim follows immediately once we observe that $\Alb_0(\bP¹,0)$ is a point. \end{proof} \begin{itemize}