Context-free pumping lemma
For every context-free language L, some p≥1 makes every w∈L with |w|≥p decomposable as w=uvxyz with |vxy|≤p, |vy|≥1 and uvⁱxyⁱz∈L for every integer i≥0.
Key knowledge
For every context-free language L, some p≥1 makes every w∈L with |w|≥p decomposable as w=uvxyz with |vxy|≤p, |vy|≥1 and uvⁱxyⁱz∈L for every integer i≥0. A repeated variable on a sufficiently deep parse-tree path permits the two surrounding portions to be pumped together.
Conditions and scope
- The decomposition is existential and can depend on w; to refute context-freeness every permitted decomposition must fail for some i.
- This necessary condition does not by itself characterize all context-free languages.
Sources · 1
References
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Evidence and review
Lecture 5 slides 3–5: quantified lemma, repeated-variable parse-tree construction, nonempty pumped portions and bounded span.
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