Two Lemmas on Formal Power Series

Authors: KAZIM İLHAN İKEDA, MASATOSHI İKEDA

Abstract: Let $L$ be a local field and $\tilde{L}$ the completion of the maximal unramified extension of $L$. In this short note, we prove \noindent (I) the sequences \noindent (1$^{\prime\prime}$) $0\rightarrow {\cal O}_L$[[$X_1,\ldots,X_n$]]$\rightarrow{\cal O}_{\tilde{L}}$[[$X_1,\ldots,X_n$]] $\stackrel{\phi_L-1}{\ra}{\cal O}_{\tilde{L}}$[[$X_1,\ldots,X_n$]]$\rightarrow 0$ \noindent and \noindent (2$^{\prime\prime}$) $1\rightarrow {\cal O}_L$[[$X_1,\ldots,X_n$]]$^x\rightarrow{\cal O}_{\tilde{L}}$[[$X_1,\ldots,X_n$]] $^x\stackrel{\phi_L-1}{\ra}{\cal O}_{\tilde{L}}$[[$X_1,\ldots,X_n$]]$\rightarrow 1$ \noindent are exact, where $\theta_L$ is the Frobenius automorphism over $L$ applied on the coefficients of ${\cal O}_{\tilde{L}}$[[$X_1,\ldots, X_n$]], and $\theta_L-1$ respectively denotes the mapping $\alpha\mapsto\alpha^{\theta L}-\alpha$ for \noindent (1$^{\prime\prime}$) and $\epsilon\mapsto\frac{\epsilon^{\phi}L}{\epsilon}$ for (2$^{\prime\prime}$); \noindent (II) ${\cal C}^{\circ}(\tilde{L},h)$ being the group the Coleman power series of degree 0, the sequence $$ 1\rightarrow {\cal C}^{\circ}(L, h)\rightarrow{\cal C}^{\circ}(\tilde{L},h)C^{\circ}(\tilde{L},h) \rightarrow 1 $$ \noindent is exact, where ${\cal C}^{\circ}(L,h)={\cal O}_L[[X]]^x\cap{\cal C}^{\circ}(\tilde{L},h)$

Keywords: Formal power series, Local fields, Formal groups, Lubin-Tate theory, Coleman power series.

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