KUO, Chung-Cheng, and Nai-Sher YEH. “Multiplicative Perturbations of Local C-Cosine Functions”. Studia Universitatis Babeș-Bolyai Mathematica, vol. 66, no. 3, Sept. 2021, pp. 575–589, doi:10.24193/subbmath.2021.3.13.