diff --git a/article_de_rham_cyclic.synctex.gz b/article_de_rham_cyclic.synctex.gz index e394342..9328542 100644 Binary files a/article_de_rham_cyclic.synctex.gz and b/article_de_rham_cyclic.synctex.gz differ diff --git a/article_de_rham_cyclic.tex b/article_de_rham_cyclic.tex index de0364c..3ce5c41 100644 --- a/article_de_rham_cyclic.tex +++ b/article_de_rham_cyclic.tex @@ -154,12 +154,14 @@ the $t$th upper (resp. lower) ramification jump of $X \to Y$ at $P$. Suppose that $\pi : X \to Y$ is a $\ZZ/p^n$-cover. Let $\langle G_P : P \in X(k) \rangle = \ZZ/p^m = G_{P_0}$ for $P_0 \in X(k)$. Then, as $k[\ZZ/p^n]$-modules: % \[ - H^1_{dR}(X) \cong J_{p^n}^{2 (g_Y - 1)} \oplus J_{p^n - p^{n-m} + 1}^2 \oplus \bigoplus_{P \neq P_0} J_{p^n - \frac{p^n}{e_{X/Y, P}}}^2 - \oplus \bigoplus_P \bigoplus_{t = 0}^{n-1} J_{p^n - p^t}^{u_{X/Y, P}^{(t+1)} - u_{X/Y, P}^{(t)}}. + H^1_{dR}(X) \cong J_{p^n}^{2 (g_Y - 1)} \oplus J_{p^n - p^{n-m} + 1}^2 \oplus \bigoplus_{P \neq P_0} J_{p^n - p^n/e_P}^2 + \oplus \bigoplus_P \bigoplus_{t = 0}^{n-1} J_{p^n - p^t}^{u_{P}^{(t+1)} - u_{P}^{(t)}}, \] + % + where $e_P := e_{X/Y, P}$ and $u_P^{(t)} := u_{X/Y, P}^{(t)}$. \end{Theorem} % -Write $H := \ZZ/p^n = \langle \sigma \rangle$. +Write $H := \langle \sigma \rangle \cong \ZZ/p^n$. For any $k[H]$-module $M$ denote: % \begin{align*} @@ -168,9 +170,9 @@ For any $k[H]$-module $M$ denote: \end{align*} % Recall that $\dim_k T^i M$ determines the structure of $M$ completely (cf. ????). -In the inductive step we use also the group $\ZZ/p^{n-1}$. In this case -we denote the irreducible $k[\ZZ/p^{n-1}]$-modules by $\mc J_1, \ldots, \mc J_{p^{n-1}}$ -and $\mc T^i M := T^i_{\ZZ/p^{n-1}} M$ for any $k[\ZZ/p^{n-1}]$-module $M$. +In the inductive step we use also the group $H' := \ZZ/p^{n-1}$. In this case +we denote the irreducible $k[H']$-modules by $\mc J_1, \ldots, \mc J_{p^{n-1}}$ +and $\mc T^i M := T^i_{H'} M$ for any $k[H']$-module $M$. Note also that for $j \ge 1$: % @@ -187,7 +189,7 @@ Note also that for $j \ge 1$: \end{itemize} -\begin{Lemma} \label{lem:G_invariants_etale} +\begin{Lemma} \label{lem:G_invariants_\'{e}tale} If the $G$-cover $X \to Y$ is \'{e}tale, then the natural map % \[ @@ -224,31 +226,72 @@ Note also that for $j \ge 1$: \end{proof} % -\begin{Lemma} - Let $M$ be a $k[H]$-module. Let $T^i M$ be as above and - $\mc T^i M := T^i_{H'} M$ for $H' \le H$, $H' \cong \ZZ/p^{n-1}$. - If $\mc T^i M \cong \mc T^{i+1} M$ for some $i$ then: +\begin{Lemma} \label{lem:lemma_mcT_and_T} + Let $M$ be a $k[H]$-module. Let $T^i M$ and $\mc T^i M$ be as above. + If $\dim_k \mc T^i M = \dim_k \mc T^{i+1} M$ for some $i$ then: % \[ - T^{pi + p} M \cong T^{pi + p - 1} M \cong \ldots \cong T^{pi - p + 1} M. + \dim_k T^{pi + p} M = \dim_k T^{pi + p - 1} M = \ldots = \dim_k T^{pi - p + 1} M. \] \end{Lemma} \begin{proof} - ?? + By Lemma~\ref{lem:TiM_isomorphism}: + % + \begin{align*} + \dim_k \mc T^i M &= \dim_k T^{pi} M + \ldots + \dim_k T^{pi - p + 1} M\\ + &\ge \dim_k T^{p^n - p^{n-1}} M + \ldots + \dim_k T^{p^n - p^{n-1}} M + = \dim_k \mc T^{p^{n-1} - p^{n-2}} M. + \end{align*} + % + Since the left-hand side and right hand side are equal, we conclude by Lemma~\ref{lem:TiM_isomorphism} \end{proof} \begin{proof}[Proof of Theorem~\ref{thm:cyclic_de_rham}] We use the following notation: $H' := \langle \sigma^p \rangle \cong \ZZ/p^{n-1}$, $H'' := H/\langle \sigma^{p^{n-1}} \rangle \cong \ZZ/p^{n-1}$, $Y' := X/H'$, $X'' := X/H''$. Write also $\mc M := H^1_{dR}(X)$. + We consider now two cases. If the cover $X \to Y$ is \'{e}tale, then by Lemma~\ref{lem:G_invariants_\'{e}tale} we have + % + \[ + \dim_k T^1 \mc M = 2 g_Y + \] + % + Moreover, by induction assumption, since $2(g_{Y'} - 1) = p \cdot 2 \cdot (g_Y - 1)$: + % + \[ + \mc M \cong \mc J_{p^{n-1}}^{2 p \cdot (g_Y - 1)} \oplus k^{\oplus 2}. + \] + % + Therefore $\dim_k \mc T^2 \mc M = \ldots = \dim_k \mc T^{p^{n-1}} \mc M = 2 p (g_Y - 1)$, + which implies that + % + \[ + \dim_k T^p \mc M = \ldots = \dim_k T^{p^n} \mc M = 2(g_Y - 1). + \] + % + by Lemma~\ref{lem:lemma_mcT_and_T}. Thus, for $i = 2, \ldots, p$: + % + \[ + \dim_k T^i \mc M \ge 2(g_Y - 1) = \dim_k T^{p+1} \mc M + \] + % + On the other hand: + % + \begin{align*} + \sum_{i = 2}^p \dim_k T^i \mc M = 2g_X - \dim_k T^1 \mc M - \sum_{i = p+1}^{p^n} \dim_k T^i \mc M = (p-1) \cdot 2(g_Y - 1). + \end{align*} + % + Thus $\dim_k T^i \mc M = 2(g_Y - 1)$ for every $i \ge 2$, which ends the proof in this case. + + Assume now that $X \to Y$ is not \'{e}tale. Therefore $X \to X''$ is also not \'{e}tale. By induction hypothesis for $H'$ acting on $X$, we have the following isomorphism of $k[H']$-modules: % \[ - \mc M \cong \mc J_{p^{n-1}}^{2 (g_{Y'} - 1)} \oplus \mc J_{p^{n-1} - p^{n - 1 -m'} + 1}^2 \oplus \bigoplus_{P \neq P_0} \mc J_{p^n - \frac{p^{n-1}}{e_{X/Y', P}}}^2 + \mc M \cong \mc J_{p^{n-1}}^{2 (g_{Y'} - 1)} \oplus \mc J_{p^{n-1} - p^{n - 1 -m'} + 1}^2 \oplus \bigoplus_{P \neq P_0} \mc J_{p^{n-1} - p^{n-1}/e'_P}^2 \oplus \bigoplus_P \bigoplus_{t = 0}^{n-1} \mc J_{p^n - p^t}^{u_{X/Y', P}^{(t+1)} - u_{X/Y', P}^{(t)}} \] % - where + where $e'_P := e_{X/Y', P}$ and % \[ m' := @@ -257,7 +300,7 @@ Note also that for $j \ge 1$: m, & \textrm{ otherwise.} \end{cases} \] - + % Therefore, for $i \le p^n - p^{n-1}$ % \begin{align*} @@ -268,30 +311,20 @@ Note also that for $j \ge 1$: \end{align*} % In particular, $\dim_k \mc T^1 \mc M = \ldots = \dim_k \mc T^{p^{n-1} - p^{n-2}} \mc M$. - On the other hand, by Lemma~\ref{lem:TiM_isomorphism}: - % - \begin{align*} - \dim_k \mc T^1 \mc M &= \dim_k T^1 \mc M + \ldots + \dim_k T^p \mc M\\ - &\ge \dim_k T^{p^n - p^{n-1}} \mc M + \ldots + \dim_k T^{p^n - p^{n-1}} \mc M - = \dim_k \mc T^{p^{n-1} - p^{n-2}} \mc M. - \end{align*} - % - Since the left-hand side and right hand side are equal, we conclude by Lemma~\ref{lem:TiM_isomorphism} - that + Thus by Lemma~\ref{lem:lemma_mcT_and_T} % \[ \dim_k T^1 \mc M = \ldots = \dim_k T^{p^n - p^{n-1}} \mc M = \frac{1}{p} \dim_k \mc T^1 \mc M. \] % - We consider now two cases. If the cover $X \to Y$ is \'{e}tale, then by Lemma~\ref{lem:G_invariants_etale} we have + By Lemma~\ref{lem:trace_surjective} since $X \to X''$ is not \'{e}tale, the map $\tr_{X/X''} : H^1_{dR}(X) \to H^1_{dR}(X'')$ is surjective. Recall that + in $\FF_p[x]$ we have the identity: % \[ - \dim_k T^1 \mc M = 2 g_{X''} + 1 + x + \ldots + x^{p-1} = (x - 1)^{p-1}. \] - - then the cover $X \to Y$ must be also \'{e}tale. - Thus the proof follows in this case by~\cite{Nakajima??Inventiones}. Suppose now that - $X \to X''$ is not \'{e}tale. Then, by Lemma~\ref{lem:trace_surjective}, the map $\tr_{X/X''} : H^1_{dR}(X) \to H^1_{dR}(X'')$ is surjective. Moreover, note that in the group ring $k[H]$ we have: + % + Therefore in the group ring $k[H]$ we have: % \[ \tr_{X/X''} = \sum_{j = 0}^{p-1} (\sigma^{p^{n-1}})^j = (\sigma^{p^{n-1}} - 1)^{p-1} = @@ -304,7 +337,7 @@ Note also that for $j \ge 1$: \ker(\tr_{X/X''} : \mc M \to \mc M'') = \mc M^{(p^n - p^{n-1})} \] % - and that $\tr_{X/X''}$ induces a $k$-linear isomorphism $T^{i + p^n - p^{n-1}} M \to \mc T^i M''$ for any $i \ge 1$. Thus: + and that $\tr_{X/X''}$ induces a $k$-linear isomorphism $T^{i + p^n - p^{n-1}} \mc M \to \mc T^i \mc M''$ for any $i \ge 1$. Thus: % \[ \dim_k T^{i + p^n - p^{n-1}} \mc M = \dim_k \mc T^i \mc M'' = .... @@ -315,7 +348,7 @@ Note also that for $j \ge 1$: \section{Hypoelementary covers} % -Assume now that $G = H \rtimes_{\chi} \ZZ/c$. +Assume now that $G = H \rtimes_{\chi} C = \langle \sigma \rangle \rtimes_{\chi} \langle \rho \rangle \cong \ZZ/p^n \rtimes_{\chi} \ZZ/c$. % \begin{Proposition} \label{prop:main_thm_for_hypoelementary} Main Theorem holds for a hypoelementary $G$ as above and $k = \ol k$. @@ -377,7 +410,7 @@ Assume now that $G = H \rtimes_{\chi} \ZZ/c$. is an isomorphism of $k$-vector spaces between $T^{i+1} \mc M$ and $T^i \mc M$ for $i = 2, \ldots, p^n$. This yields an isomorphism of $k[C]$-modules for $i \ge 2$: % - \begin{equation} \label{eqn:TiM=T1M_chi_etale} + \begin{equation} \label{eqn:TiM=T1M_chi_\'{e}tale} T^i \mc M \cong (T^2 \mc M)^{\chi^{-i+2}} \end{equation} % @@ -394,10 +427,10 @@ Assume now that $G = H \rtimes_{\chi} \ZZ/c$. % Thus, since by induction hypothesis $\mc T^i \mc M$ is determined by ramification data, we have by Lemma~\ref{lem:N+Nchi+...} that $T^2 \mc M$ is determined by ramification data. - Moreover, by Lemma~\ref{lem:G_invariants_etale}, $T^1 \mc M \cong H^1_{dR}(X'')$ + Moreover, by Lemma~\ref{lem:G_invariants_\'{e}tale}, $T^1 \mc M \cong H^1_{dR}(X'')$ is also determined by ramification data (???). - Assume now that $X \to Y$ is not etale. Analogously as in the previous case, Lemma~\ref{lem:TiM_isomorphism_hypoelementary} and proof of Theorem~\ref{thm:cyclic_de_rham} + Assume now that $X \to Y$ is not \'{e}tale. Analogously as in the previous case, Lemma~\ref{lem:TiM_isomorphism_hypoelementary} and proof of Theorem~\ref{thm:cyclic_de_rham} yield an isomorphism of $k[C]$-modules: % \begin{equation} \label{eqn:TiM=T1M_chi}