# How to prove every context-free language over a unary alphabet is regular?

How can I show that every context-free language over a unary alphabet is regular?

• May I make a minor, stylistic recommendation? I suggest phrasing the question as "How do I show that X?" rather than "Show that X." When I see a question that belongs "Show that...", my immediate reaction is -- "No, you show it". "Show that..." sounds demanding, like you are commanding us to do something. Also, general advice: if you want help figuring out how to prove something, you should show us what you've tried so far, where you've gotten stuck, and where you ran into the problem and what the context is. – D.W. Oct 22 '13 at 7:08
• This is a problem statement, not a question. – Raphael Oct 28 '13 at 8:04
• Well, it is word-for-word copied from Problem 3 from Problem Set 4 in Computer Science 121 at Harvard, though to be fair to the OP, the problem set was due 16 days before the question was posted. :) – Ray Toal Dec 20 '15 at 22:18

The following proof follows Pighizzini, Shallit and Wang, Unary Context-Free Grammars and Pushdown Automata, Descriptional Complexity and Auxiliary Space Lower Bounds.

Let $L$ be a unary context-free language. Assume for simplicity that $\epsilon \notin L$, and consider a grammar $G = \langle V,\{a\},P,S \rangle$ for $L$ in Chomsky normal form. Denote $h = |V|$.

In the sequel, whenever we say parse tree, we mean parse tree in $G$.

Let $\Pi$ be the collection of all triples $(U,i,j)$ such that $U$ is a parse tree rooted at some nonterminal $A$ which represents a derivation of the sentential form $a^i A a^j$, where $0 < i+j < 2^h$.

Let $(U,i,j) \in \Pi$, and let $A$ be the label of the root of $U$. Given a parse tree $S$ containing a node $v$ labeled $A$, we can "pump" $S$ by $U$ by replacing $v$ with a copy of $U$, attaching the children of $v$ to the leaf of $U$ labeled $A$.

Lemma. If $\ell > 2^{h-1}$ and $a^\ell \in L$ and $T$ is a parse tree for $a^\ell$, then there exists a triple $(U,i,j) \in \Pi$ and a parse tree $S$ for $a^{\ell-i-j}$ such that $T$ is obtained from $S$ by pumping by $U$.

Proof. Since $\ell > 2^{h-1}$, $T$ must have depth at least $h+1$ (recall that the last level corresponds to productions of the form $A \to a$), and so a path of length $h+1$ edges. This path contains $h+1$ nonterminals, one of which must repeat. Consider such a repetition within the last $h+1$ nonterminals of the path. The repetition corresponds to a triple $(U,i,j) \in \Pi$ (note $i+j < 2^h$ since we chose a repetition within the last $h+1$ nonterminals, and $i+j > 0$ since the grammar is in Chomsky normal form). By "pumping out" this derivation, we obtain the parse tree $S$ for $a^{\ell-i-j}$. $\quad\square$

Corollary. Every parse tree in $L$ can be obtained in the following way:

• Start with a parse tree for some $a^\ell$, where $\ell \leq 2^{h-1}$.
• Repeatedly pump by $U$ for some $(U,i,j) \in \Pi$.

Using this, we can construct an NFA for $L$:

• Guess a parse tree for some $a^\ell$, where $\ell \leq 2^{h-1}$, and read the word $a^\ell$.
• Set $X$ to be the set of nonterminals appearing in the parse tree.
• Perform the following operation an arbitrary number of times:
1. Guess $(U,i,j) \in \Pi$ such that the label of the root of $U$ appears in $X$.
2. Read $a^{i+j}$.
3. Add all nonterminals in $U$ to $X$.

This shows that $L$ is regular.

Parikh's theorem can be proved in the same way.

You could use the more general fact (due to Parikh) that the commutative image of a context-free language of $A^*$ is a rational subset of $\mathbb{N}^{|A|}$. For a unary alphabet, this gives your statement.

This was my first attempt at it:

First let $L$ be our context free language. Using the pumping lemma for context free grammars: the pumping constant is $p$ and $m \ge p$.

We have a string $s = 1^m = uvwxy$, and we say $a_m = |uwy|$ and $b_m = |vx|$ such that $s = 1^{a_m}1^{b_m}$ where $1 \le b_m \le p$

Because our language was context free, we can say this about a string. Now let us define two more languages:

$$Mod = \{m \in \mathbb{N} | 1^m \in L\}$$

$$L' = \{x \in L| |x| < p\}$$

Now we can construct out language $L$ from of finite union of regular languages, meaning it is regular:

$$L = L' \cup \bigcup_{m \in Mod} 1^{a_m}1^{b_m} = L' \cup \bigcup_{m \in Mod} 1^{a_m}(1^{b_m})^*$$

• The last union is over $m\in Mod$ which without further explanations still is an infinite union. – Hendrik Jan Oct 21 '13 at 22:54