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The component group of a Sato-Tate group $G$ is the quotient $G/G^0$ of $G$ by its identity component $G^0$, which is a normal subgroup of finite index.

If $G$ is the Sato-Tate group of a motive defined over a number field $K$, then the component group $G/G^0$ is canonically isomorphic to the Galois group of a finite extension $L/K$. For the motive attached to an abelian variety $A$ of dimension at most 3, $L$ is the smallest field over which all endomorphisms of $A_{\overline{K}}$ (the base change of $A$ to an algebraic closure of $K$) are defined.

Component groups are named according to their isomorphism type. This does not determine them uniquely; one can obtain a more explicit description by examining the list of generators. Common notations used in component group names include:

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• Last edited by Andrew Sutherland on 2021-01-01 15:15:08
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