I have questions about how to put the grammar below in CNF - Chomsky Normal Form:

S ->aAa | bBb | ВВ;

A -> C;

B -> S | A;

C -> S | ε;

I did it like this:

  1. I eliminated empty productions:

S ->aAa | aa | bBb | bb| ВВ;

A -> C;

B -> S | A;

C -> S;

  1. I eliminated unit productions:

S ->aAa | aa | bBb | bb | ВВ;

A -> aAa | aa | bBb | bb | ВВ;

B -> Aa | aa | bBb | bb | ВВ;

C -> Aa | aa | bBb | bb | ВВ;

  1. I eliminated the useless variables (C), but it only becomes inaccessible after removing the unitary production of A ->C). Can anyone help me and tell me if this is right?

1 Answer 1


It is the case that $C$ becomes inaccessible, but your grammar isn't in normal form (yet). Rules like $S \to aAa$ or $S \to bb$ aren't allowed in CNF.

Also your grammar isn't equal to the original, since you forgot that $S$ is nullable when removing the $\varepsilon$ production (e.g. you could derive $S \Rightarrow BB \Rightarrow^* AA \Rightarrow^* CC \Rightarrow^* \varepsilon$ which isn't possible in your last grammar).

When bringing a grammar in CNF it's often a good idea to remove all $A \to B$ transitions first, and then remove the $\varepsilon$ transitions. So first start off by adding a new start variable $S'$

$$S \to S'$$ $$S' \to aAa | bBb | BB$$ $$A \to C$$ $$B \to S' | A$$ $$C \to S' | \varepsilon$$

then remove the $A \to C$ and $B \to A$ transition

$$S \to S'$$ $$S' \to aAa | bBb | BB$$ $$A \to S' | \varepsilon$$ $$B \to S' | \varepsilon$$

$C$ is now inaccessible, so we'll ignore it. Remove $A \to S'$ and $B \to S'$

$$S \to S'$$ $$S' \to aAa | aS'a | bBb | bS'b | BB | S'B | BS' | S'S'$$ $$A \to \varepsilon$$ $$B \to \varepsilon$$

remove the $\varepsilon$ transitions

$$S \to S' | \varepsilon$$ $$S' \to aa | aS'a | bb | bS'b | S' | S'S' | \varepsilon$$

remove $S' \to \varepsilon$ and $S' \to S'$

$$S \to S' | \varepsilon$$ $$S' \to aa | aS'a | bb | bS'b | S'S'.$$

This can now be brought into CNF

$$S \to S' | \varepsilon$$ $$S' \to AA | AX | BB | BY | S'S'$$ $$A \to a$$ $$X \to S'A$$ $$B \to B$$ $$Y \to S'B$$

as you should verify :]


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