Automata Theory Introduction - Automata Theory

Automata What is it?

The word "Automata" is depicted from the Greek word "αὐτόματα" which coveys "self-acting". An automaton (Automata in plural) is known as an abstract self-propelled computing device to be determined to operate sequence of operations automatically.
A Finite Automaton (FA) or Finite State Machine (FSM) is known as an automaton with a finite number of states.

Formal definition of a Finite Automaton

An automaton is determined by a 5-tuple (Q, ∑, δ,q0, F), where −
  • Q is a finite set of states.
  • ∑ is a finite set of symbols, called the alphabet of the automaton.
  • δ is the transition function.
  • q0 is the initial state from where any input is processed (q0∈ Q).
  • F is a set of final state/states of Q (F ⊆ Q).

Related Terminologies


  • Definition − An alphabet is any finite set of symbols.
  • Example − ∑ = {a, b, c, d} is an alphabet set where ‘a’, ‘b’, ‘c’, and ‘d’ are symbols.


  • Definition − A string is a finite sequence of symbols taken from ∑.
  • Example − ‘cabcad’ is a valid string on the alphabet set ∑ = {a, b, c, d}

Length of a String

  • Definition − It is the number of symbols present in a string. (Denoted by |S|).
  • Examples
    • If S = ‘cabcad’, |S|= 6
    • If |S|= 0, it is called an empty string (Denoted by λ or ε)

Kleene Star

  • Definition − The Kleene star, ∑*, is called as a unary operator on a set of symbols or strings, ∑, that gives the infinite set of all possible strings of all possible lengths over ∑including λ.
  • Representation ∑* = ∑0∪ ∑1∪ ∑2∪…….where ∑p is the set of all possible strings of length p.
  • Example − If ∑ = {a, b}, ∑* = {λ, a, b, aa, ab, ba, bb,………..}

Kleene Closure / Plus

  • Definition − The set ∑+ is the infinite set of all possible strings of all possible lengths over ∑ excluding λ.
  • Representation − ∑+= ∑1∪ ∑2∪ ∑3∪…….
+= ∑* − { λ }
  • Example − If ∑ = { a, b } , ∑+ = { a, b, aa, ab, ba, bb,………..}


  • Definition − A language is a subset of ∑* for some alphabet ∑. It can be finite or infinite.
  • Example − If the language takes all possible strings of length 2 over ∑ = {a, b}, then L = { ab, bb, ba, bb}

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