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For the alphabet $Σ$={a,b,c}

I was wondering how you would say:

T that has elements from Σ, so could be T=a, T=bc

I was considering maybe $Σ^*$ or $Σ^+$ would describe that, but I am not sure what both of them mean.

I guess $Σ^*$={aa, aaa, ba, abc, ...} and not sure what $Σ^+$ is.

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Usually, $\Sigma^*$ denotes the set of all strings of length zero or more over the alphabet $\Sigma$. Thus, $\Sigma^* = \{\epsilon, a, b, c, aa, ab, ac, ba, bb, bc, ca, cb, cc, aaa, \ldots\}$, where $\epsilon$ denotes the empty string.

$\Sigma^+$ is the set of all strings of length $1$ or more over the alphabet $\Sigma$. Thus, $\Sigma^+ := \Sigma^* - \{\epsilon\} = \{a,b,c,aa,\ldots\}$.

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