A note on d'Alembert's functional equation
Annales mathématiques Blaise Pascal, Tome 8 (2001) no. 1, pp. 1-6. 
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Akkouchi, Mohamed. A note on d'Alembert's functional equation. Annales mathématiques Blaise Pascal, Tome 8 (2001) no. 1, pp. 1-6. http://archive.numdam.org/item/AMBP_2001__8_1_1_0/

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