I am working on a pumping lemma question and trying to prove that the following is not context-free, but I can't finish the proof. The language is

$$L = \{ a^n b^m \mid n \leq m^2 \}$$

Assume Demon picks $p$ and I choose $s = a^{p^2}b^p$ and $s = uvwxy$ such that $p \geq |vwx|$.

The first case is $vwx = b^j$. If we choose $i = 0$ everything is ok and the relationship isn't right.

The second case is $vwx = a^j$. If we choose $i = 2$ every thing is ok.

I get stuck when $vwx = a^j b^k$. How can I finish the proof in this case?

Did I choose the right string? I repeated them for $s = a^{p}b^p$ but it didn't work.

  • 1
    $\begingroup$ Is the condition $n \le m^2$ or $n \ge m^2$? $\endgroup$ May 14 at 16:54
  • $\begingroup$ @YuvalFilmus I think it is indeed $n\leq m^2$, since it is coherent with the first two cases: removing $b$'s prove the word is not in $L$, and it is the same when adding $a$'s. $\endgroup$
    – Nathaniel
    May 14 at 17:58
  • $\begingroup$ @YuvalFilmus it's n≤m2. question edited. $\endgroup$
    – hermi
    May 15 at 7:31

When $vwx = a^j b^k$, there are two possibilities:

  • $v$ contains both $a$s and $b$s, or $x$ contains both $a$s and $b$s. In this case $uv^2wx^2y \notin a^*b^*$, and in particular $uv^2wx^2y \notin L$.
  • $v = a^s$ and $x = b^t$, where $s,t>0$ and $s+t \leq p$ (since $|vwx| \leq p$). In this case, $uv^0wx^0y = a^{p^2-s}b^{p-t}$, and so $p^2 - s \leq (p-t)^2 \leq (p-1)^2$, which translates to $s \geq 2p-1$, which is impossible, since $s \leq p-1$.
  • $\begingroup$ I don't understand the first one. We are not sure about the amounts of a or b in v and x. $\endgroup$
    – hermi
    May 18 at 6:42
  • $\begingroup$ They don't matter. What matters is that after pumping, you're not of the form $a^*b^*$. $\endgroup$ May 18 at 6:46
  • $\begingroup$ for example v = a and x = abbb when i = 2 uv^2wx^2y is a member of language . $\endgroup$
    – hermi
    May 18 at 6:48
  • $\begingroup$ What word do you start with? And what does $uv^2wx^2y$ look like? $\endgroup$ May 18 at 6:49
  • $\begingroup$ Can you give an example? I say it's not a form of {a^nb^m∣n≤m2} $\endgroup$
    – hermi
    May 18 at 6:50

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