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I'm looking for the name of this theorem:

Let $P$, $Q$ be languages. Let $X$ be a language variable. Then the language equation $X=PX + Q$ (here $+$ denotes union) has a solution $X=P^*Q$, and the solution is unique if the null string doesn't belong to $P$.

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Arden's Rule.

(That was my answer, but the rules of Stackexchange force me to continue to at least 30 characters)

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