References
- J. Aczél, Lectures on Functional Equations and Their Applications, Mathematics in Science and Engineering, Vol. 19, Academic Press, New York, 1966.
- 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.
- 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.
- B. Ebanks, The sine addition and subtraction formulas on semigroups, Acta Math. Hungar. 164 (2021), no. 2, 533–555.
- B. Ebanks, The cosine and sine addition and subtraction formulas on semigroups, Acta Math. Hungar. 165 (2021), no. 2, 337–354.
- 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.
- A. Jafar, O. Ajebbar, and E. Elqorachi, A Kannappan-sine addition law on semigroups, arXiv preprint, 2023. Available at arXiv: 2304.14146.
- 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.
- Pl. Kannappan, A functional equation for the cosine, Canad. Math. Bull. 11 (1968), no. 3, 495–498.
- T.A. Poulsen and H. Stetkær, The trigonometric subtraction and addition formulas, Aequationes Math. 59 (2000), no. 1–2, 84–92.
- H. Stetkær, Functional Equations on Groups, World Scientific Publishing Company, Singapore, 2013.
- H. Stetkær, The cosine addition law with an additional term, Aequationes Math. 90 (2016), no. 6, 1147–1168.
- H. Stetkær, Kannappan’s functional equation on semigroups with involution, Semigroup Forum 94 (2017), no. 1, 17–30.