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I have an odd PDA problem that I cant seem to construct. I haven't come across one like this before.

$L = \{w\in\{a,b\}^{*} : 3\#_{a}(w) \leq 5\#_{b}(w) \leq 4\#_{a}(w)\}$

Could I get some pointers on how to tackle such a problem like this so I can be more familiar with the approaches here.

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  • $\begingroup$ $3n_a(w)$ means the number of $a$'s in the string? I saw a different notation, where # means the number of characters, e.g. 3*#a $\le$ 5*#b. $\endgroup$ – Evil Oct 29 '16 at 2:07
  • $\begingroup$ Yes you are correct. $\endgroup$ – James Combs Oct 29 '16 at 2:07
  • $\begingroup$ I cant seem to find a pattern that always works. Much less turn this into a grammar to construct a PDA from. In order to initially satisfy the constraints, there must either be 0 $a$'s and $b$'s or at least 3 $a$'s and 2 $b$'s. But then afterwards, I cant find a steady pattern. $\endgroup$ – James Combs Oct 29 '16 at 2:16
  • $\begingroup$ it looks like a pattern emerges. once you have 3 $a$'s and 2 $b$'s, increases both by 1 works until you have 5 $a$'s. Then you must increase the $a$'s by 2 before you increase the $b$'s by 1. Then increase both by 1 again until reaching 10 $a$'s. At which point you increase the $a$'s by 2 before increasing the $b$'s by 1 again. Im not sure if it continues this way. Not enough room to try more lol. $\endgroup$ – James Combs Oct 29 '16 at 2:25
  • $\begingroup$ I have spent too much time on this haha. Moving on $\endgroup$ – James Combs Oct 29 '16 at 2:59
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It's your exercise, so you should solve it yourself... but I will give you two hints.

  1. Study the methods used in Grammar for a language with 1/3 of a's. They are helpful.

  2. Use non-determinism. Build a non-deterministic PDA. (How do you think non-determinism might help here?)

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  • $\begingroup$ Thank you. Exactly what I asked for. I appreciate not giving me the answer $\endgroup$ – James Combs Oct 29 '16 at 6:17

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