Repetitive strings are not context-free
RAIRO. Informatique théorique, Tome 16 (1982) no. 3, pp. 191-199. 
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Ross, Rockford; Winklmann, Karl. Repetitive strings are not context-free. RAIRO. Informatique théorique, Tome 16 (1982) no. 3, pp. 191-199. http://archive.numdam.org/item/ITA_1982__16_3_191_0/

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