APPROXIMATION OF TWO GENERAL FUNCTIONAL EQUATIONS IN 2-BANACH SPACES.

In this paper, we study the Ulam stability and hyperstability of two general functional equations in several variables in 2-Banach spaces. Multi-additive and multi-Jensen functions are particular cases of these functional equations. We also improve the main results of Theorem 3 and Theorem 4 of [Ciepli'nski, K. Ulam stability of functional equations in 2-Banach spaces via the fixed point method. J. Fixed Point Theory Appl. 23 (2021), no. 3, Paper No. 33, 14 pp.] and their consequences. [ABSTRACT FROM AUTHOR]