How can I describe a Push Down Automaton that recognize the language $P=\{a^{2^n} | n \geq 0 \}$?
My approach
I know that the language can be described by a Turing Machine, but how i can the stack recognize that language?
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1This is impossible. You can show it is not context-free by applying the Pumping Lemma. – Hendrik Jan Oct 01 '17 at 14:13
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Existence of a TM accepting a language $L$ does not guarantee existence of a PDA accepting this language. Please look here. – fade2black Oct 01 '17 at 14:16