Mach, Andrzej, and Zenon Moszner. “Unstable (Stable) System of Stable (unstable) Functional Equations”. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica 9 (March 12, 2010): 43–48. Accessed October 30, 2026. https://studmath.uken.krakow.pl/article/view/13244.