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I'm trying to construct a context free grammar for the language $\{a^ib^j:2i\neq3j+1\}$. I found that when $j$ is even, there are no problems. When $j$ is odd, there are some constraints on $i$. Now these are some examples of words that may NOT be parsed by the CFG:

  • aab
  • aaaaabbb (5 a's and 3 b's)
  • aaaaaaaabbbbb (8 a's and 5 b's)
  • ...

But I'm completely stuck on finding a CFG for this language. Are there any tips that might help me in finding CFG's for a more complex language? I'm able to find the CFG for simpler languages, but I find this one particularly hard.

Pieter Verschaffelt
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1 Answers1

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Try constructing CFGs for the two related languages $$ \{a^ib^j : 2i > 3j + 1\}, \quad \{a^ib^j : 2i < 3j+1\}. $$ This can be accomplished by constructing a grammar for $$ \{a^ib^j : 2i = 3j + 1\} $$ and some related languages.

Yuval Filmus
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