Counter automaton

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In computer science, a counter automaton is a Pushdown automaton with only two symbols, A and the initial symbol in \Gamma\, (the finite set of stack symbols). This class of automata can recognize a subset of Context free languages, for instance the language:

 \{\ a^nb^n : n \in \mathbb{N} \}

To accept the above language, let x be a word on the form above. The automaton can use the symbol A to count the number of a's in x (writing an A for each a in x) and deleting an A for each b in x.