# On $L^* - \{\epsilon\} = L^+$

$$\Sigma^* - \{\epsilon\} = \Sigma^+$$

$$L^* - \{\epsilon\} = L^+$$

Which of the above is always true?

I was following a discussion on a site and I came across this question. Some fellow scholars claimed that the second statement is not always true. The answer they provided is this:

$$S_1$$: $$Σ^* – \{ϵ\} = Σ^+$$: TRUE // Always true, definition of $$Σ^+$$

$$S_2$$: $$L^* – \{ϵ\} = L^+$$: FALSE // May or may not be true

False when $$ϵ$$ belongs to $$L$$, then $$L^+$$ and $$L^*$$ will both contain $$ϵ$$.

PS: In $$S_2$$, it depends purely on the given language.

But I have this doubt:

How can $$L^+$$ contain $$ϵ$$? we obtain $$L^+$$ by deleting $$ϵ$$ from $$L^*$$, and in $$L^*$$ of the language where $$L=\{ϵ\}$$, $$L^*$$ will only contain $$ϵ$$, so if we remove $$ϵ$$ from it, then the language should become $$\emptyset$$, which is nothing but $$L^+$$ here. So shouldn't the second statement also be true?

Say $$L^+ = \{ϵ,ϵϵ,ϵϵϵ,\dots\}$$.

Now an important point. Note that we say Set of all strings, now in set theory duplicates are not allowed, moreover $$ϵ= ϵϵ =ϵϵϵ =\dots$$. They are all just the same. So $$L^∗$$ should be only $$\{ϵ\}$$, rather than $$\{ϵ,ϵ,ϵϵ,ϵϵϵ,\dots\}$$, right?

• Usual definition is $L^+ = L \cdot L^*$. – Dmitri Urbanowicz Aug 30 at 11:28
• see my doubt above – HIRAK MONDAL Aug 30 at 12:10

You got the definition of $$L^+$$ wrong. It is not $$L^* \setminus \{\epsilon\}$$. Rather, it is $$L^+ = \bigcup_{n=1}^* L^n.$$ You can check that $$\epsilon \in L^+$$ iff $$\epsilon \in L$$. Therefore:
• If $$\epsilon \notin L$$ then $$L^+ = L^*\setminus\{\epsilon\}$$.
• If $$\epsilon \in L$$ then $$L^+ = L^*$$.