Embeddings in iteration groups and semigroups with nontrivial units. Jürgen Weitkämper (1983) Stochastica
Étude sur les équations fonctionnelles A. Grévy (1896) Annales scientifiques de l'École Normale Supérieure
Existence of solutions for equations involving iterated functional series. Murugan, V., Subrahmanyam, P.V. (2005) Fixed Point Theory and Applications [electronic only]