Automorphisms of formal power series under substitution
Benjamin Muckenhoupt · Transactions of the American Mathematical Society · 1961
For a simpler presentation see H. Kneser, Reele analytischer Lösungen der Gleichung (j>[(j>(z)] =e! und verwandter funklionalgleichungen, J. Reine Angew.Math.vol.187 (1949) pp.56-67; see p. 58.(2) This result was announced for the case when the base field is the complex numbers by N. J. Fine and Bertram Kostant in an abstract, The group of formal power series under iteration, Bull.Amer.Math.Soc.vol.61 (1955) pp.36-37.Their proof has not yet appeared.