... linear mappings by linear mappings,” Journal ofFunctional Analysis, vol. 46, no. 1, pp. 126–130, 1982.17 J. M. Rassias, “On approximation of approximately linear mappings by linear mappings,” ... the sine and cosine functional equations,” Proceedings of the AmericanMathematical Society, vol. 110, no. 1, pp. 109–115, 1990.24 J. Chang and J. Chung, “The stability of the sine and cosine ... unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.1. Introduction In 1,Acz´el and Chung introduced the following functional equation:lj1fjαjx...