Let $Y$ be a smooth projective complex curve of genus 2 and $f : X \to Y$ a finite etale cover. Choose two distinct points A and B on $Y$ and let $\Sigma\subset X$ be the set of complex points $P$ such that $f(P)$ is A or B. For any $x\in \Sigma$ choose an element $L(x) \in Pic(Y)[2]$. Does there exist an element $L\in Pic(X\times Y)[2]$ such that, for every $x \in \Sigma$, the restriction of $L$ to $x\times Y = Y$ is equal to $L(x)$?

Angelo says that the restriction of $L$ to $x\times Y = Y$ is constant. Is it so and why?

OK. Thank you for the answer.

  • $\begingroup$ What is the relationship between $x \in \Sigma$ and $L(x)$? $\endgroup$ – Simon Rose Dec 8 '19 at 8:29
  • $\begingroup$ None. Another way to phrase the question: is the restriction map $Pic(X\timesY)[2] \ra Pic(\Sigma\times Y)[2]$ surjective? $\endgroup$ – user95246 Dec 8 '19 at 8:50
  • 2
    $\begingroup$ Given an element $L\in Pic(X\times Y)[2]$, the associated map $X to Pic(Y)[2]$ is constant. $\endgroup$ – Angelo Dec 8 '19 at 8:53

If $Y$ is a complex projective algebraic variety, the Picard group $\operatorname{Pic}Y$ has the structure of an algebraic variety; if $X$ is another algebraic variety, any line bundle $L$ gives a regular map $X \to \operatorname{Pic}Y$ by sending $x \in X$ to the class of $L \mid x\times Y$. If $L$ is 2-torsion, then the map $X \to \operatorname{Pic}Y$ has image contained in $\operatorname{Pic}(Y)[2]$; but $\operatorname{Pic}(Y)[2]$ is finite, so if $X$ is connected, the map has to be constant.


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