Here, I read this:

For all CFL, there is a one-state PDA that recognizes this language.

What is the proof/idea behind this claim?

CFL: Context Free Languages PDA: Push Down Automaton

  • 1
    $\begingroup$ Check here. $\endgroup$
    – Nathaniel
    Feb 19 at 13:35

1 Answer 1


We have to be precise.

Each context-free language can be accepted by empty stack using a push-down automaton with a single state, or by final state and two states. (In the latter case we obviously need one accepting and one non-accepting state.)

The proof uses the equivalence between context-free grammars and push-down automata. Every CFG is equivalent to a PDA with a single state (and empty stack acceptance). Wikipedia mentions the expand-match construction which directly simulates the context-free derivation on the stack.


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