# Regular expression notation clarification

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.

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, $$\Sigma^+ := \Sigma^* - \{\epsilon\} = \{a,b,c,aa,\ldots\}$$.