Construct a PDA, which recognizes the following language $L$: $L = \{w\;|\; |w|_a > 2|w|_b\}$, so it is the language that consists of words which have more than twice as many $a$'s as $b$'s.

I have constructed a PDA, which I believe to recognize $L$, however it is too complicated to prove its correctness. Is there an easy way to construct such an automata? I need a hint.

  • $\begingroup$ Take a look at automata accepting identical numbers, then $|w|_a > |w|_b$ and $|w|_a = |w|_b = 2n, n \in \mathbb N$. $\endgroup$
    – greybeard
    Nov 2, 2021 at 5:31

1 Answer 1


While reading prefixes $u$ of a word $w$, you need to count $|u|_a - 2|u|_b$ and check whether this value is greater or equal to $1$ when $u = w$.

Counting only the value of $|u|_a$ is easy: when you read a $a$, push a $A$ in the stack. The number of $A$ in the stack will be the same as $|w|_a$ at the end.

Counting only the value of $-2|u|_b$ can be done with the same idea: when you read a $b$, push two $B$'s in the stack.

However, if you need to do both, it could be a bit trickier than that. You could still combine the ideas:

  • if there is nothing or a $A$ at the top of the stack while reading a $a$, then push one more $A$ in the stack;
  • if there is nothing or a $B$ at the top of the stack while reading a $b$, then push two more $B$'s in the stack;

Can you find what to do if there is a $A$ at the top while reading a $b$, or a $B$ while reading a $a$? I will edit my answer if you need more details.

  • $\begingroup$ I think that we should somehow transform sequence in stack into equivalent one, but more simple, for example if we have $aaba$ then after reading first two letters there are two $a$'s in stack but after adding $b$ (which is equivalent to adding $-2$) we can substitute these two $a$'s by $\varepsilon$. But I don't get why we should push two more A's in the stack in your first case, shouldn't it be the other way round? $\endgroup$
    – giochi
    Nov 2, 2021 at 12:08
  • $\begingroup$ Yeah, my mistake. $\endgroup$
    – Nathaniel
    Nov 2, 2021 at 14:03
  • $\begingroup$ @Egor Consider accepting the answer if it helped you. $\endgroup$
    – Nathaniel
    Nov 28, 2022 at 8:11

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