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Chomsky Hierarchy represents the class of languages that are accepted by the different machines.

Hierarchy of grammars according to Chomsky is explained below as per the grammar types −

**Type 0** − It is an Unrestricted grammars

Unrestricted grammar − an **unrestricted grammar** is a 4-tuple (T,N,P,S), which consisting of −

T = set of terminals

N = set of nonterminal

P =as set of productions, of the form −

v->w

where v and w are strings consisting of nonterminal and terminals.

S = is called the **start symbol**.

**Example** − Turing Machine (TM)

**Type 1** − Context-sensitive grammars

All productions are of form,

v -> w where |v| < |w|

uAv -> uwv with w != epsilon,

I.e., A -> w but only in the context of u _ v.

A context-sensitive grammar is equivalent to a **linear bounded** and **context-sensitive language**.

**Example ** − Linear Bounded Automaton (LBA)

**Type 3** − Context-free grammars −

All productions are of the form −

A -> x − where A is nonterminal, x is a string of nonterminal and terminals, A context-free grammar is equivalent to a **pushdown automaton (PDA)** and to **context free languages**.

**Example** − Pushdown Automaton (PDA)

**Type 3** − Regular grammars

All productions are of the form −

A -> xB

A -> x where A, B are nonterminal, x is a string of terminals.

Regular grammars are equivalent to **regular sets** and to **finite automata**.

**Example ** − Finite Automaton (FA)

Grammar type | Grammar accepted | Language accepted | Automaton |
---|---|---|---|

Type 0 | unrestricted grammar | recursively enumerable language | Turing Machine |

Type 1 | context-sensitive grammar | context-sensitive language | linear-bounded automata |

Type 2 | Context-free grammar | Context-free language | Push down automata |

Type 3 | regular grammar | regular language | finite state automaton |

The Chomsky hierarchy is depicted in the diagram given below −

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