Pumping lemma

IN the theory of formal languages in computability theory, a pumping lemma or pumping argument states that, for a particular language to be a member of a language class, any sufficiently long string in the language contains a section, or sections, that can be removed, or repeated any number of times, with the resulting string remaining in that language. The proofs of these lemmas typically require counting arguments such as the pigeonhole principle.

Tree Automata Techniques and Applications

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2. ^ T. Hayashi (1973). "On Derivation Trees of Indexed Grammars - An Extension of the uvxyz Theorem". Publication of the Research Institute for Mathematical Sciences (Research Institute for Mathematical Sciences) 9 (1): 61–92.