John M. Rassias, M. Arunkumar, S. Ramamoorthi and S. Hemalatha (2014) “Ulam - Hyers stability of a 2- variable AC - mixed type functional equation in quasi - beta normed spaces: direct and fixed point methods”, Malaya Journal of Matematik. India, 2(02), pp. 108–128. doi: 10.26637/mjm202/003.