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Automata Theory

A study of * The theory of formal languages * The mathematical modeling of systems * The computability of a problem/function * What is a computer

Alphabets (Σ)

A finite set of symbols.


A finite sequence of symbols from an alphabet.

  • Empty string: ε
  • Concatination: ω=abd, α=ce, ωα=abdce
  • Exponentiation: ω=abd, ω^3=abdabdabd, ω^0=ε
  • Reversal: ω=abd, ω^R = dba
  • Σ^k = set of all k-length strings formed by the
  • symbols in Σ
  • Σ^1 != Σ
  • Kleene Closure: Σ* = Σ^0 ∪ Σ^1 ∪ Σ^2 ∪… = ∪^k≥0 Σ^k
  • Σ+ = Σ* - ε


A set of strings over an alphabet. φ != {ε}.

Finite Automata / Finite State Machines

Deterministic Finite Automata

(Q, Σ, δ, q0, F)

  • Q: finite set of states
  • Σ:finite input alphabet
  • δ: transition function (Q × Σ -> Q)
  • q0: initial state (only 1)
  • F: set of final states (0 or more)
  • one transition only for each input symbol from each state.

Transition Table

Transition Diagram

Languages Accepted by DFAs

The language contains L(M) all input strings accepted by L.

  • There exist languages which are not Regular: There is no DFA that accepts such a language.

Nondeterministic Finite Accepter (NFA)

  • The lambda symbol never appears on the input tape
  • NFAs are interesting because we can express languages easier than DFAs
  • NFAs accept the Regular Languages


M1 = M2 iff L(M1) = L(M2)