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Apr 8, 2016 at 10:41 comment added Mark Wildon That was my guess, and it still looks plausible to me now. But I don't have a proof.
Apr 8, 2016 at 7:05 comment added js21 Should I understand that if $f = (1-p(X+X^{-1})^{-1}$, then $f_+$ is not in $\mathbb{Z}_p\{X\}$ ?
Apr 8, 2016 at 6:57 comment added js21 You can look into Raynaud's "Anneaux henseliens". If $B$ is the integral closure of $\mathbb{Z}_p[X]$ in $\mathbb{Z}_p \langle X \rangle$, then $\mathbb{Z}_p \{ X\}$ is the localization of $B$ w.r.t $1 + B \cap p\mathbb{Z}_p \langle X \rangle$. The same construction applies to $\mathbb{Z}_p [X,X^{-1} ]$. In particular $(1-p(X+X^{-1})^{-1}$ is in $\mathbb{Z}_p\{X,X^{-1}\}$.
Apr 6, 2016 at 19:23 comment added Mark Wildon Please could you give a reference for the definitions of $\mathbb{Z}_p\{X\}$ and $\mathbb{Z}_p\{X,X^{-1}\}$, or include them in the question? (I am not an expert, so am guessing slightly what Henselization means in this context.) Specifically, is is true that $(1-p(X+X^{-1})^{-1}$ is in $\mathbb{Z}_p\{X,X^{-1}\}$?
Apr 4, 2016 at 9:57 history asked js21 CC BY-SA 3.0