References
- R. Cooper, On a duplication formula, Math. Gaz. 41 (1957), pp. 217–218.
- H. Farzadfard, Periodic solutions of the Poincaré functional equation, J. Math. Anal. Appl. DOI:10.1016/j.jmaa.2021.124933.
- H. Farzadfard and B. Khani Robati, The structure of disjoint group of continuous functions, Abstr. Appl. Anal. 2012 (2012), Article ID 790758, pp. 1–14.
- H.G. Forder, Duplication formulae, Math. Gaz. 41 (1957), pp. 215–217.
- M. Kuczma, On a characterization of the cosine, Ann. Pol. Math. 16 (1964), pp. 53–57.
- H. Poincaré, Sur une classe étendue de transcendantes uniformes, C. R. Acad. Sci. Paris 103 (1886), pp. 862–864.
- H. Poincaré, Sur une classe nouvelle de transcendantes uniformes, J. Math. Pures Appl. IV. Ser. 6 (1890), pp. 316–365.
- A.N. Šarkovski, Characterization of the cosine (in Russian), Aequ. Math. 9 (1973), pp. 121–128.