Formal power series and some theorems of J. F. Ritt in arbitrary characteristic

We study composition of power series and polynomials over algebraically clo sed fields of arbitrary characteristic. The so-called Boettcher function of a power series is introduced and investigated. It is the principal aim of this paper to prove some results going back to J. F. Ritt in this general s etting. In particular, we determine the pairs of permutable polynomials and characterize polynomials which satisfy a certain rational functional equat ion and Polynomials which have a common iterate.