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I am currently studying materials for my uni subject. There are two examples of Kleene algebras, but I don't see what is the difference between them.

  • Class ${2^{\Sigma}}^{*}$ of all subsets of $\Sigma^{*}$ with constants $\emptyset$ and $\{\varepsilon\}$ and operations $\cup$, $\cdot$ and $*$.
  • Class of all regular subsets of $\Sigma^{*}$ with constants $\emptyset$ and $\{\varepsilon\}$ and operations $\cup$, $\cdot$ and $*$.

What is the difference between ${2^{\Sigma}}^{*}$ and all regular subsets of $\Sigma^{*}$? What is that difference I don't see? Thanks in advance.

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The difference between the two is that $2^{\Sigma^*}$ contains all languages over $\Sigma$, whereas the class of all regular subsets of $\Sigma^*$ contains only the regular languages over $\Sigma^*$. So if $L$ is a non-regular language over $\Sigma$ then $L \in 2^{\Sigma^*}$ while $L$ doesn't belong to the set of all regular languages over $\Sigma$. For any $\sigma \in \Sigma$, you can take $L = \{ \sigma^{n^2} : n \geq 0 \}$, for example.

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  • $\begingroup$ Thank you. I didn't think of this that way. Somehow, I automatically assumed only regular languages. My bad. $\endgroup$ – Marek Milkovič Oct 18 '15 at 7:10

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