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I have some difficulties proving next statement :

If $A$ is a block for a group $G$ and $a \in A$, show that $A$ is a union of orbits for $G_a$ (where $G_a$ is a stabilizer of a in G ).

I would be very thankful for some advice how to start.

Thanks!

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    I'm unfamiliar with the term "block". Could you specify what the definition is? Also, could make more explicit the fact that the group $G$ is acting on the set $A$?2011-06-06
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    @Zev: If $G$ acts on a set $S$, then a subset $A$ of $S$ is a block if for every $g\in G$ either $gA=A$, or $gA\cap A = \emptyset$.2011-06-06
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    @Arturo: Ah, thanks.2011-06-06

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