When $(D[[X]])_{P[[X]]}$ is a valuation ring

Abstract:

Let D be an integral domain with identity and let K denote the quotient field of D. If P is a prime ideal of D denote by $P[[X]]$ that prime ideal of $D[[X]]$ consisting of all those formal power series each of whose coefficients belongs to P. In this paper the following question is considered: When is ${(D[[X]])_{P[[X]]}}$ a valuation ring? Our main theorem states that if ${(D[[X]])_{P[[X]]}}$ is a valuation ring, then ${D_P}$ must be a rank one discrete valuation ring. Moreover, we show that if ${D_P}$ is a rank one discrete valuation ring and if $PD[[X]] = P[[X]]$, then ${(D[[X]])_{P[[X]]}}$ is a valuation ring. We also give an example to show that ${(D[[X]])_{P[[X]]}}$ need not be a valuation ring when ${D_P}$ is rank one discrete.