# Are abelian varieties degree two covers of some projective space

Let $A$ be an abelian variety over a field $k$ of dimension $g\geq 2$.

There exists a finite morphism $A\to \mathbf{P}^g_k$. Here's the question.

Does there exist a finite morphism $A\to \mathbf{P}^g_k$ of degree two?

Can we say something about the minimal degree of a finite morphism $A\to \mathbf{P}^g_k$?

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A better analogue of the degree 2 map $E\to\mathbb{P}^1$ for an elliptic curve is the degree 2 map $A\to A/\pm1$ from $A$ to the associated Kummer variety. (Of course, you need to blow up some points if you want a smooth quotient, and then then map is only rational, it's not a morphism.) –  Joe Silverman Aug 1 '12 at 14:05

For a very general, principally polarized Abelian variety $(A,\Theta)$ of dimension $g$ over $\mathbb{C}$, every Cartier divisor $D$ on $A$ is numerically equivalent to $m\Theta$ for some integer $m$. In particular, the intersection number $D^g$ is $m^g \Theta^g$. So the minimal degree of an effective, nonzero divisor is $g!$, not $2$.
Your argument works for any abelian variety of dimension at least $3$: by Riemann-Roch, $g!$ divides $D^g$ for any ample divisor $D$. One can also eliminate the $g=2$ case using the fact that any double cover of $\mathbb{P}^2$ with trivial canonical bundle is a $K3$ surface. –  ulrich Aug 1 '12 at 13:33