diff --git a/blueprint/src/chapter/FujisakiProject.tex b/blueprint/src/chapter/FujisakiProject.tex index 5c0e4c10..d04da86a 100644 --- a/blueprint/src/chapter/FujisakiProject.tex +++ b/blueprint/src/chapter/FujisakiProject.tex @@ -70,6 +70,7 @@ \section{Enter the adeles} \label{AdeleRing.DivisionAlgebra.compact_quotient} If $D$ is a division algebra then the quotient $D^\times\backslash D_{\A}^{(1)}$ with its quotient topology coming from $D_{\A}^{(1)}$, is compact. + \uses{DivisionAlgebra.units_cocompact} \end{theorem} The rest of this miniproject is devoted to a proof of this theorem. @@ -116,6 +117,7 @@ \section{The proof} $\beta X\cap D^\times\not=\emptyset$. \end{lemma} \begin{proof} + \uses{E} Indeed by the previous lemma, the map $\beta E\to D\backslash D_{\A}$ isn't injective, so there are distinct $\beta e_1,\beta e_2\in \beta E$ with $e_i\in E$ and @@ -130,6 +132,7 @@ \section{The proof} $X\beta^{-1}\cap D^\times\not=\emptyset$. \end{lemma} \begin{proof} + \uses{E, addHaarScalarFactor.left_mul_eq_right_mul} Indeed, $\beta^{-1}\in D_{\A}^{(1)}$, and so left multiplication by $\beta^{-1}$ doesn't change Haar measure on $D_{\A}$, so neither does right multiplication (by theorem~\ref{addHaarScalarFactor.left_mul_eq_right_mul}). @@ -142,7 +145,9 @@ \section{The proof} \label{Y_meet_units_D} $Y\cap D^\times$ is finite. \end{lemma} -\begin{proof} It suffices to prove that $Y\cap D$ is finite. +\begin{proof} + \uses{E} + It suffices to prove that $Y\cap D$ is finite. But $D\subseteq D_{\A}$ is a discrete additive subgroup, and hence closed. And $Y\subseteq D_{\A}$ is compact. So $D\cap Y$ is compact and discrete, so finite. @@ -152,14 +157,16 @@ \section{The proof} of $D_{\A}$, and define $K:= (T^{-1}.X) \times X\subset D_{\A}\times D_{\A}$, noting that $K$ is compact because $X$ is compact and $T$ is finite. -\begin{lemma} For every $\beta\in D_{\A}^{(1)}$, there exists $b\in D^\times$ +\begin{lemma} + \label{exists_product} + For every $\beta\in D_{\A}^{(1)}$, there exists $b\in D^\times$ and $\nu\in D_{\A}^{(1)}$ such that $\beta=b\nu$ and $(\nu,\nu^{-1})\in K.$ \end{lemma} \begin{proof} - - By an earlier lemma, $\beta X\cap D^\times\not=\emptyset$, and by another earlier lemma, - $X\beta^{-1}\cap D^\times\not=\emptyset$, so we can write $\beta x_1=b_1$ - and $x_2\beta^{-1}=b_2$ with obvious notation. + \uses{E, X_meets_kernel, X_meets_kernel'} + By lemma~\ref{X_meets_kernel}, $\beta X\cap D^\times\not=\emptyset$, + and lemma~\ref{X_meets_kernel'}, $X\beta^{-1}\cap D^\times\not=\emptyset$, + so we can write $\beta x_1=b_1$ and $x_2\beta^{-1}=b_2$ with obvious notation. Multiplying, $x_2x_1=b_2b_1\in Y\cap D^\times=T$ (recall that $Y=X*X$ and $T=Y\cap D^\times$ is finite); call this element $t$. @@ -175,28 +182,28 @@ \section{The proof} We can now prove Fujisaki's theorem. \begin{theorem} - \label{DivisionAlegbra.units_cocompact} + \label{DivisionAlgebra.units_cocompact} $D^\times\backslash D_{\A}^{(1)}$ is compact. \end{theorem} \begin{proof} + \uses{DistribHaarChar.continuous, exists_product, Y_meet_units_D} Indeed, if $M$ is the preimage of $K$ under the map $D_{\A}^{(1)} \to D_{\A}\times D_{\A}$ sending $\nu$ to $(\nu,\nu^{-1})$, then $M$ is a closed subspace of a compact space so it's compact (note that $\delta_{D_{\A}}$ is continuous, by theorem~\ref{DistribHaarChar.continuous}). - The previous lemma shows that $M$ surjects onto + Lemma~\ref{exists_product} shows that $M$ surjects onto $D^\times\backslash D_{\A}^{(1)}$ which is thus also compact. \end{proof} We note here a useful consequence. \begin{theorem} - \label{DivisionAlegbra.units_cocompact'} + \label{DivisionAlgebra.units_cocompact'} $D^\times\backslash(D\otimes_K\A_K^\infty)^\times$ is compact. \end{theorem} -\begin{remark} In this generality the quotient might not be Hausdorff. -\end{remark} \begin{proof} + \uses{DivisionAlgebra.units_cocompact} There's a natural map $\alpha$ from $D^\times\backslash D_{\A}^{(1)}$ to $D^\times\backslash (D\otimes_K \A_K^\infty)^\times$. We claim that it's surjective. Granted this claim, we are home, because if we put the quotient @@ -216,3 +223,5 @@ \section{The proof} as multiplication by $x$ is just scaling by a factor of $x$ on $D_{\R}\cong\R^d$. In particular we can set $x=y^{1/d}$. \end{proof} +\begin{remark} In this generality the quotient might not be Hausdorff. +\end{remark}