Let R be a commutative
ring with identity, R[t] the polynomial ring in an indeterminate t over R, and
R[t][[X]] the formal power series ring in an indeterminate X over R[t]. Let
α =∑i=1∞ai(t)Xi and β =∑i=0∞bi(t)Xi be elements of R[t][[X]] where ai(t) and
bi(t) are elements of R[t] for each i. This paper gives necessary and sufficient
conditions in order that there exist an R-automorphism of R[t][[X]] mapping t and X
onto α and β respectively.