Hi,

Let $\Delta$ be a quaternion algebra over $\mathbf Q$ and let $\mathcal O_\Delta$ be a maximal order in $\Delta$. Recall that a false elliptic curve over a scheme $S$ is a pair $(A/S,i)$ consisting of an abelian surface $A/S$ and a ring homomorphism $i : \mathcal O_\Delta\to End_S(A)$. Suppose that $\Delta$ is indefinite, i.e., $\Delta\otimes\mathbf R \simeq M_2(\mathbf R)$. There is an involution $*:\Delta\to\Delta$ that coincides with taking transpose under the previous isomorphism. It is well-known (easy?) that if $S$ is a $\mathbf Q$-scheme there is a polarization of $A/S$ such that the corresponding Rosatti involution in $End_S(A)\otimes\mathbf Q$ corresponds to $* : \Delta\to\Delta$ by $i$. Moreover, this polarization is unique up to a rational number.

Question 1: is it possible to find a (necessarily unique) principal polarization with this property?

Question 2: is it possible to find such a principal polarization over a general scheme $S$?

Thanks!

Hi,

Let $\Delta$ be a quaternion algebra over $\mathbf Q$ and let $\mathcal O_\Delta$ be a maximal order in $\Delta$. Recall that a false elliptic curve over a field $K$ is a pair $(A/K,i)$ consisting of an abelian surface $A/K$ and a ring homomorphism $i : \mathcal O_\Delta\to End_K(A)$. Suppose that $\Delta$ is indefinite, i.e., $\Delta\otimes\mathbf R \simeq M_2(\mathbf R)$. There is an involution $*:\Delta\to\Delta$ that coincides with taking transpose under the previous isomorphism. It is well-known (easy?) that if $K$ is of characteristic zero there is a polarization of $A/K$ such that the corresponding Rosatti involution in $End_K(A)\otimes\mathbf Q$ corresponds to $* : \Delta\to\Delta$ by $i$. Moreover, this polarization is unique up to a rational number.

Question 1: is it possible to find a (necessarily unique) principal polarization with this property?

Question 2: is it possible to find such a (principal) polarization over a field $K$ of non-zero characteristic?

Thanks!