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  "body": "\n## Summary\n\nIn mathematics and group theory, a **block** for the action of a group $G$ on a [[set|0BSJ7RQ9BW]] $X$ is a subset $Y$ of $X$ whose images under $G$ either coincide with $Y$ or are disjoint from $Y$. These images form a **block system**, a partition of $X$ that is $G$-invariant. In terms of the associated [[equivalence relation|0G8VEZ3WTK]] on $X$, $G$-invariance means that\n",
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