Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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 $P$.

share|cite|improve this question

Arden's Rule.

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

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.