# Greibach normal form

A formal grammar $G=(\Sigma,N,P,\sigma)$ is said to be in Greibach normal form if every production has the following form:

 $A\to aW$

where $A\in N$ (a non-terminal symbol), $a\in\Sigma$ (a terminal symbol), and $W\in N^{*}$ (a word over $N$).

A formal grammar in Greibach normal form is a context-free grammar. Moreover, any context-free language not containing the empty word $\lambda$ can be generated by a grammar in Greibach normal form. And if a context-free language $L$ contains $\lambda$, then $L$ can be generated by a grammar that is in Greibach normal form, with the addition  of the production $\sigma\to\lambda$.

Let $L$ be a context-free language not containing $\lambda$, and let $G=(\Sigma,N,P,\sigma)$ be a grammar in Greibach normal form generating $L$. We construct a PDA $M$ from $G$ based on the following specifications:

1. 1.

$M$ has one state $p$,

2. 2.

the input alphabet of $M$ is $\Sigma$,

3. 3.

the stack alphabet of $M$ is $N$,

4. 4.

the initial stack symbol of $M$ is the start symbol $\sigma$ of $G$,

5. 5.

the start state of $M$ is the only state of $M$, namely $p$

6. 6.

there are no final states,

7. 7.

the transition function $T$ of $M$ takes $(p,a,A)$ to the singleton $\{(p,W)\}$, provided that $A\to aW$ is a production of $G$. Otherwise, $T(p,a,A)=\varnothing$.

It can be shown that $L=L(M)$, the language  accepted on empty stack, by $M$. If we further define $T(p,\lambda,\sigma):=\{(p,\lambda)\}$, then $M$ accepts $L\cup\{\lambda\}$. As a result, any context-free language is accepted by some PDA.

## References

• 1 J.E. Hopcroft, J.D. Ullman, , Addison-Wesley, (1969).
Title Greibach normal form GreibachNormalForm 2013-03-22 18:55:22 2013-03-22 18:55:22 CWoo (3771) CWoo (3771) 7 CWoo (3771) Definition msc 68Q42 msc 68Q45 GNF ChomskyNormalForm KurodaNormalForm