The **endomorphism algebra** of an abelian surface $A$ over a field $K$ is the $\Q$-algebra $\End(A) \otimes \Q$. This is a special case of the endomorphism algebra of an abelian variety.

When $A$ is an abelian surface over $\Q$, there are five possibilities for $\End(A)\otimes\Q$:

- $\Q$;
- a real quadratic field (in which case $A$ has
**real multiplication**, denoted RM); - an imaginary quadratic field (in which case $A$ has
**complex multiplication**, denoted CM); - $\Q\times \Q$;
- $\mathrm{M}_2(\Q)$;

The first three cases occur when $A$ is simple, while the last two cases occur when $A$ is isogenous to a product of elliptic curves $E_1\times E_2$ over $\overline K$. Which of the last two cases occurs depends on whether $E_1$ and/or $E_2$ are isogenous or not.

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- Review status: reviewed
- Last edited by Edgar Costa on 2020-10-21 14:35:14

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