Explicit Expressions for Iterates of Power Series

Abstract: In this paper, we present five different formulas for both discrete and fractional iterations of an invertible power series $f$ utilizing a novel and unifying approach from umbral calculus. Established formulas are extended, and their proofs simplified, while new formulas are introduced. In particular, through the use of $q$-calculus identities, we eliminate the requirement for $f'(0)$ to equal $1$ and, consequently, the corresponding new expressions for the iterative logarithm are derived.