A non-context-free equal-block witness
The language {0ⁿ1ⁿ2ⁿ:n≥0} is not context free.
Key knowledge
The language {0ⁿ1ⁿ2ⁿ:n≥0} is not context free. For a proposed pumping length p, choose 0ᵖ1ᵖ2ᵖ; a span vxy of length at most p cannot contain both a 0 and a 2. Pumping down removes at least one symbol from at most two blocks while leaving the third block's count p, violating equality.
Conditions and scope
- The argument covers every decomposition allowed by the context-free pumping lemma.
- The witness shows a PDA/CFG limitation; a Turing machine or a context-sensitive model can still decide this language.
Sources · 1
References
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Evidence and review
Lecture 5 slide 6 all-decompositions proof for three equal blocks, with i=0 pumping-down count argument made explicit.
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Entry details
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- university-core.1
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- University core expansion