Let [tex]F[/tex] be a field. Let [tex] c \in F[/tex]. I am trying to show that if [tex] c = f^2[/tex] where [tex]f\in F[[x]][/tex], then [tex]f\in F[/tex]. So I am able to get rid of the terms with negative exponent. So now I'm left with a formal power series. Anyone knows how to do this? Thanks!