Faà di Bruno's formula and nonhyperbolic fixed points of one-dimensional maps. Ponomarenko, Vadim (2004) International Journal of Mathematics and Mathematical Sciences
Funktionalgleichungen für konstante Funktionen. Peter Volkmann, Norbert Steinmetz (1984) Aequationes mathematicae
Further results on stability of normal regionas for linear homogeneous functional equations. M. Czerni (1995) Aequationes mathematicae