User:Jshflynn/Kleene Star is Linguistic Structure
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Theorem
Let $\Sigma$ be an alphabet.
Let $\Sigma^{*}$ be the Kleene star of $\Sigma$ and $\circ$ denote concatenation.
Then $(\Sigma^{*}, \circ)$ is a linguistic structure.
Proof
As $\Sigma^{*} \subseteq \Sigma^{*}$ we have that $\Sigma^{*}$ is a formal language over $\Sigma$.
From Kleene Plus is Linguistic Structure we have that $\circ$ is closed on $\Sigma^{*}-\{\lambda \}$.
Including $\lambda$ in the underlying set we have from Empty Word is Two-sided Identity that $\Sigma^{*}$ is closed under $\circ$ and hence $(\Sigma^{*}, \circ)$ is a linguistic structure.
$\blacksquare$