diff --git a/blueprint/src/chapter/QuaternionAlgebraProject.tex b/blueprint/src/chapter/QuaternionAlgebraProject.tex index 9eb446a..dd63cc5 100644 --- a/blueprint/src/chapter/QuaternionAlgebraProject.tex +++ b/blueprint/src/chapter/QuaternionAlgebraProject.tex @@ -162,16 +162,16 @@ \section{Statement of the main result of the miniproject} The finite-dimensionality theorem is in fact an easy consequence of a finiteness assertion which is valid in far more generality, namely for division algebras over number fields. -We state and prove this result in this generality. Let $K$ be a number field and let $D/K$ -be a finite-dimensional central simple $K$-algebra. Assume furthermore that $D$ is a -\emph{division algebra}, that is, that every nonzero element of $D$ is a unit. The finiteness +We state and prove this result in this generality. Let $K$ be a number field and let $B/K$ +be a finite-dimensional central simple $K$-algebra. Assume furthermore that $B$ is a +\emph{division algebra}, that is, that every nonzero element of $B$ is a unit. The finiteness theorem we want is this. \begin{theorem} \lean{DivisionAlgebra.finiteDoubleCoset} \label{DivisionAlgebra.finiteDoubleCoset} - If $U\subseteq (D\otimes_F\A_F^\infty)^\times$ is a compact open subgroup, - then the double coset space $D^\times\backslash(D\otimes_F\A_F^\infty)^\times/U$ is a + If $U\subseteq (B\otimes_K\A_K^\infty)^\times$ is a compact open subgroup, + then the double coset space $B^\times\backslash(B\otimes_K\A_K^\infty)^\times/U$ is a finite set. \end{theorem} @@ -179,22 +179,32 @@ \section{Statement of the main result of the miniproject} ``a standard consequence of results about automorphic forms'', but in John Voight's book~\cite{voightbook} this is Main Theorem 27.6.14(b) and Voight calls it Fujisaki’s lemma. -Let's prove finite-dimensionality of the space of quaternionic modular forms +Let's prove Theorem~\ref{TotallyDefiniteQuaternionAlgebra.AutomorphicForm.finiteDimensional}, +the finite-dimensionality of the space of quaternionic modular forms, assuming Fujisaki's lemma. \begin{proof} \proves{TotallyDefiniteQuaternionAlgebra.AutomorphicForm.finiteDimensional} \uses{DivisionAlgebra.finiteDoubleCoset} Choose a set of coset representative $g_1,g_2,\ldots,g_n$ for $D^\times\backslash(D\otimes_F\A_F^\infty)^\times/U$. My claim is that - the function $S_{W,\chi}(U;K)\to W^n$ sending $f$ to $(f(g_1),f(g_2),\ldots,f(g_n))$ - is injective and $K$-linear, which suffices by finite-dimensionality of $W$. - $K$-linearity is easy, so let's talk about injectivity. + the function $S_{W,\chi}(U;k)\to W^n$ sending $f$ to $(f(g_1),f(g_2),\ldots,f(g_n))$ + is injective and $k$-linear, which suffices by finite-dimensionality of $W$. + $k$-linearity is easy, so let's talk about injectivity. - Say $f_1$ and $f_2$ are two elements of $S_{W,\chi}(U;K)$ which agree on + Say $f_1$ and $f_2$ are two elements of $S_{W,\chi}(U;k)$ which agree on each $g_i$. It suffices to prove that $f_1(g)=f_2(g)$ for all $g\in(D\otimes_F\A_F^\infty)^\times$. So say $g\in(D\otimes_F\A_F^\infty)^\times$, and write $g=\delta g_iu$ for $\delta \in D^\times$ and $u\in U$. Then $f_1(g)=f_1(\delta g_iu)=\delta\cdot f_1(g_i)$ (by hypotheses (i) and (iii) - of the definition of $S_{W,\chi}(U;K)$), and similarly $f_2(g)=\delta\cdot f_2(g_i)$ + of the definition of $S_{W,\chi}(U;k)$), and similarly $f_2(g)=\delta\cdot f_2(g_i)$ and because $f_1(g_i)=f_2(g_i)$ by assumption, we deduce $f_1(g)=f_2(g)$ as required. \end{proof} + +It thus remains to prove Fujisaki's lemma. + +% \section{Proof of Fujisaki's lemma} + +% We need to talk about the full adele ring of the number field~$K$. This is +% the product of the finite adele ring $\A_K^\infty$ and the ring of ``infinite adeles'' +% $K\otimes_{\Q}\R$, which is often described as $\prod_{v\infty}K_v$, where +% $v$ runs through the infinite places of $K$.