1. A Collection of Mathematical Problems;Ulam,1961
2. On the stability of the linear functional equation;Hyers;Proc. Natl. Acad. Sci. USA,1941
3. Ulam Stability of Operators, A Volume in Mathematical Analysis and Its Applications;Brzdȩk,2018
4. Functional Equations and Inequalities in Several Variables;Czerwik,2002
5. Hyers–Ulam stability of functional equations in several variables;Forti;Aequationes Math.,1995