# Prove that $\{0^n 1^{n\cdot m} : n,m \in \mathbb{N}\}$ is not context-free

This is a homework problem I have spent several hours on. A "hint" is given that we may use this fact: If $n,j,k \in \mathbb{N}$ satisfy $n \geq 2$ and $1 \leq j+k \leq n$, then $n^2+j$ does not evenly divide $n^3+k$.

I cannot find any way to apply this fact. It leads me to believe I should use the string $0^{p^2}1^{p^3}$ or something like that, but I am really just not sure. The pumping lemma has given me trouble since the non regular language version.

Even small hints greatly appreciated at this point.

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## migrated from cstheory.stackexchange.comFeb 15 '13 at 16:03

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Have you looked at our reference question? –  Raphael Feb 16 '13 at 14:42

If you use the pumping lemma on the word $w=0^{p^2} 1^{p^3}$, consider the partition $w=xyzuv$, where $|yzu|\le n$ and $|yu|>0$ ($n$ being the length of $w$). It is easy to prove that from all the cases (that is, from all the possibilities for $y$ and $u$), the only non-trivial case is when $y=0^i$ and $u=1^j$, in which case the hint you mentioned finishes the job.