New generalizations of the Poisson kernel. Haruki, Hiroshi, Rassias, Themistocles M. (1997) Journal of Applied Mathematics and Stochastic Analysis
Non-negative continuous solutions of a functional inequality Marek Kuczma (1979) Annales Polonici Mathematici
Note on the differentiability of solutions of class Cr of a functional equation with respect to a parameter. S. Czerwik (1975) Aequationes mathematicae
Note on the existence of continuous solutions of a functional equation of n-th order Karol Baron (1974) Annales Polonici Mathematici