J. Aczél, Lectures on Functional Equations and Their Applications, Mathematics in Science and Engineering, Vol. 19, Academic Press, New York, 1966.
Google Scholar
O. Ajebbar and E. Elqorachi, The cosine-sine functional equation on a semigroup with an involutive automorphism, Aequationes Math. 91 (2017), no. 6, 1115–1146.
Google Scholar
O. Ajebbar and E. Elqorachi, Solutions and stability of trigonometric functional equations on an amenable group with an involutive automorphism, Commun. Korean Math. Soc. 34 (2019), no. 1, 55–82.
Google Scholar
B. Ebanks, The sine addition and subtraction formulas on semigroups, Acta Math. Hungar. 164 (2021), no. 2, 533–555.
Google Scholar
B. Ebanks, The cosine and sine addition and subtraction formulas on semigroups, Acta Math. Hungar. 165 (2021), no. 2, 337–354.
Google Scholar
B. Ebanks, Around the sine addition law and d’Alembert’s equation on semigroups, Results Math. 77 (2022), no. 1, Paper No. 11, 14 pp.
Google Scholar
A. Jafar, O. Ajebbar, and E. Elqorachi, A Kannappan-sine addition law on semigroups, arXiv preprint, 2023. Available at arXiv: 2304.14146.
Google Scholar
S. Kabbaj, M. Tial, and D. Zeglami, The integral cosine addition and sine subtraction laws, Results Math. 73 (2018), no. 3, Paper No. 97, 19 pp.
Google Scholar
Pl. Kannappan, A functional equation for the cosine, Canad. Math. Bull. 11 (1968), no. 3, 495–498.
Google Scholar
T.A. Poulsen and H. Stetkær, The trigonometric subtraction and addition formulas, Aequationes Math. 59 (2000), no. 1–2, 84–92.
Google Scholar
H. Stetkær, Functional Equations on Groups, World Scientific Publishing Company, Singapore, 2013.
Google Scholar
H. Stetkær, The cosine addition law with an additional term, Aequationes Math. 90 (2016), no. 6, 1147–1168.
Google Scholar
H. Stetkær, Kannappan’s functional equation on semigroups with involution, Semigroup Forum 94 (2017), no. 1, 17–30.
Google Scholar