Given an alphabet $\Sigma$, $P (\Sigma^*)$, the power set of $\Sigma^*$, is a monoid, with language concatenation as morphism.

  1. What is the identity: the empty language, or the language consisting of only the empty string?

  2. What is the difference between concatenating a language with the empty language and with the language consisting of only the empty string?


  • $\begingroup$ I thought that language concatenation is defined pointwise. Shouldn't $L\emptyset=\emptyset$, then? $\endgroup$ – R. Burton Jun 23 '20 at 23:52
  • $\begingroup$ What is the definition of language concatenation? $\endgroup$ – Tim Jun 24 '20 at 0:08

For languages $L_1$ and $L_2$, the concatenation of $L_1$ with $L_2$ is defined elmentwise as:

$$L_1L_2=\{s_1s_2\mid s_1\in L_1\land s_2\in L_2\}$$

(at least in every case that I have seen)

The concatenation of a language $L$ with the empty languae is thus:

$$L\emptyset = \{s_1s_2\mid s_1\in L\land s_2\in\emptyset\}$$

By definition, $x\notin\emptyset$ for all $x$, so $(s_1\in L)\land (s_2\in \emptyset) \equiv (s_1\in L)\land \bot \equiv \bot$. Hence $L\emptyset=\{s_1s_2\mid\bot\}=\emptyset$ for any language $L$.

It stands to reason that the identity element of $\mathcal{P}(\Sigma^*)$ would be $E=\{\varepsilon\}$, where $\varepsilon$ is the empty string. This follows from $LE=EL=L$ for all $L\in\mathcal{P}(\Sigma^*)$.

For a more "mathy" take, consider the following:

Claim: If $\mathcal{P}(\Sigma^*)$ with language concatenation is a monoid, then $E=\{\varepsilon\}$ is the identity element.

proof-sketch: By definition, if $M$ is a monoid, then $M$ contains a unique identity element $e$ satisfying $xe=ex=x$ for all $x\in M$. Observe that for all $s\in\Sigma$, $s\varepsilon=\varepsilon s=s$ where $\varepsilon$ is the empty string.

If $E=\{\varepsilon\}$ is the language containing the empty string, then for all $L\in\mathcal{P}(\Sigma^*)$...

$$LE=\{le\mid l\in L\land e\in E\}=\{l\varepsilon\mid l\in L\}=\{l\mid l\in L\}=L$$


$$EL=\{el\mid e\in E\land l\in L\}=\{\varepsilon l\mid l\in L\}=\{l\mid l\in L\}=L$$



Thus, $E$ is the unique identity of the monoid $P(\Sigma^*)$. Q.E.D.


This is not a specific property of free monoids, it holds for any monoid. More precisely, take a monoid $M$ with identity $1$. Then ${\cal P}(M)$, the power set of $M$ is a monoid under the product defined, for each $S,T \in {\cal P}(M)$ by $$ ST = \{st \mid s \in S, t \in T\} $$ The identity of ${\cal P}(M)$ is the singleton $\{1\}$, since, according to the definition of the product $$ S\{1\} = \{st \mid s \in S, t \in \{1\} \} = \{s1 \mid s \in S\} = S $$ and $\{1\}S = S$ by a dual argument. The empty set is a zero of the monoid ${\cal P}(M)$. Indeed $$ S \emptyset = \{st \mid s \in S, t \in \emptyset\} = \emptyset $$ and $\emptyset S = \emptyset$ by a dual argument.

  • $\begingroup$ Thanks. Given a monoid M, does the product of two subsets S and T of M belong to P(M)? Is the product the same or different from Cartesian product of two sets? I guess yes, and think that the Cartesian product of two subsets of M doesn't belong to P(M) but to P(M*M). $\endgroup$ – Tim Jun 24 '20 at 9:45
  • $\begingroup$ Yes, $ST$ belongs to ${\cal P}(M)$ and $S \times T$ to ${\cal P}(M \times M)$. $\endgroup$ – J.-E. Pin Jun 24 '20 at 9:59

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