Geodesic
A Cosine Functional Equation with Restricted Argument
Canadian mathematical bulletin, Tome 17 (1974) no. 4, pp. 505-509

Voir la notice de l'article provenant de la source Cambridge University Press

We name a functional equation with restricted argument one in which at least one of the variables is restricted to a certain discrete subset of the domain of the other variable(s). In particular, the subset may consist of a single element.The purpose of this paper is to present a functional equation satisfied only by cosine functions. 
Etigson, L. B. A Cosine Functional Equation with Restricted Argument. Canadian mathematical bulletin, Tome 17 (1974) no. 4, pp. 505-509. doi: 10.4153/CMB-1974-089-x
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