1. Is there a sequence on four symbols in which no two adjacent segments are permutations of one another?;Brown;Amer. Math. Monthly,1971
2. Some unsolved problems;Erdös;Magyar Tud. Akad. Mat. Kutató Int. Közl.,1961
3. Remarks on the work of Axel Thue on sequences;Hedlund;Nordisk Mat. Tidskr.,1967
4. Non-repetitive sequences;Pleasants,1970
5. Über unendliche Zeichenreihen;Thue;Norske Vid. Selsk. Skr., I. Mat.-Nat. Kl. (Christiania),1906