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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{group of ideles} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{algebra}{}\paragraph*{{Algebra}}\label{algebra} [[!include higher algebra - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{ProductFormula}{Product formula}\dotfill \pageref*{ProductFormula} \linebreak \noindent\hyperlink{StrongApproximationTheorem}{Strong approximation theorem for the idele class group}\dotfill \pageref*{StrongApproximationTheorem} \linebreak \noindent\hyperlink{automorphic_forms_and_relation_to_dirichlet_characters}{Automorphic forms and Relation to Dirichlet characters}\dotfill \pageref*{automorphic_forms_and_relation_to_dirichlet_characters} \linebreak \noindent\hyperlink{FunctionFieldAnalogy}{Function field analogy}\dotfill \pageref*{FunctionFieldAnalogy} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The \emph{group of ideles} $\mathbb{I}$ is the [[group of units]] in the [[ring of adeles]] $\mathbb{A}$: \begin{displaymath} \mathbb{I} = \mathbb{A}^\times \coloneqq GL_1(\mathbb{A}) \,. \end{displaymath} In classical [[algebraic number theory]] one embeds a [[number field]] into the [[cartesian product]] of its [[completions]] at its [[archimedean absolute values]]. This embedding is very useful in the [[proofs]] of several fundamental [[theorems]]. However, it was noticed by [[Claude Chevalley]] and [[André Weil]] that the situation was improved somewhat if the number field is embedded in the cartesian product of its [[formal completions]] at all of its [[absolute values]]. With a few additional restrictions, these objects are known as the \emph{[[adeles]]}, and the [[group of units|units]] of this ring are called the \emph{ideles}. When considering the adeles and ideles, it is their [[topology]] as much as their algebraic structure that is of interest. Many important results in [[number theory]] translate into simple statements about the topologies of the adeles and ideles. For example, the finiteness of the [[ideal class group]] and the [[Dirichlet unit theorem]] are equivalent to a certain quotient of the ideles being compact and discrete. (\hyperlink{Weston}{Weston, p. 1}) \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} \begin{defn} \label{}\hypertarget{}{} The [[group of units]] of the ring of adeles $\mathbb{A}_{\mathbb{Q}}$ is called the \emph{group of ideles} \begin{displaymath} \mathbb{I}_{\mathbb{Q}} \coloneqq GL_1(\mathbb{A}_{\mathbb{Q}}) = \mathbb{A}_{\mathbb{Q}}^\times \,. \end{displaymath} It is a [[topological group]] via identification with the set $\{(x, x^{-1}) \in \mathbb{A}_\mathbb{Q}^2: \; x \in \mathbb{I}_\mathbb{Q}\}$, seen as a [[subspace]] of $\mathbb{A}_\mathbb{Q}^2$. \end{defn} \begin{remark} \label{}\hypertarget{}{} The topology on $\mathbb{I}_\mathbb{Q}$ is strictly finer than the subspace topology inherited from $\mathbb{A}_\mathbb{Q}$. For example, the set $\mathbb{R}^\times \times \prod_p \mathbb{Z}_p^\times$ is a neighborhood of $1$ in $\mathbb{I}_\mathbb{Q}$, but not in the subspace topology. Cf. the discussion \href{https://math.stackexchange.com/a/145452/43208}{here}. Note: multiplicative inversion is not continuous in the subspace topology. \end{remark} The same definition holds for the [[ring of adeles]] of any other [[global field]] $K$, here one writes \begin{displaymath} \mathbb{I}_K \coloneqq GL_1(\mathbb{A}_K) \end{displaymath} or similar. The notation $J_K$ is also common. \begin{defn} \label{IdeleClassGroup}\hypertarget{IdeleClassGroup}{} The [[quotient]] \begin{displaymath} K^\times \backslash \mathbb{I}_K = GL_1(K)\backslash GL_1(\mathbb{A}_K) \end{displaymath} is called the \emph{idele class group} of $K$. \end{defn} \begin{remark} \label{}\hypertarget{}{} The idele class group, def. \ref{IdeleClassGroup}, appears prominently in the description of the \href{moduli+space+of+bundles#OverCurvesAndTheLanglandsCorrespondence}{moduli space of line bundles} over the [[arithmetic curve]] on which $K$ is the [[rational functions]]. From there it appears in the abelian [[Langlands correspondence]] and in the abelian case of [[Tamagawa measures]]. \end{remark} The idele class group is a key object in \emph{[[class field theory]]}. \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{ProductFormula}{}\subsubsection*{{Product formula}}\label{ProductFormula} Recall the [[p-adic norm]] ${\vert -\vert}_p$ on $\mathbb{Q}$ for $p$ a [[prime number]], given by \begin{displaymath} {\left \vert \frac{a}{b}p^\ell \right \vert}_p \coloneqq p^{-\ell} \end{displaymath} for $a,b$ coprime to $p$. The usual [[absolute value]] [[norm]] one writes \begin{displaymath} {\vert -\vert}_\infty \end{displaymath} and associates with the ``[[place at infinity|prime at infinity]]''. When an index runs over the set of all primes (``finite primes'') union with the ``prime at infinity'' one usually writes it ``$v$'' instead of $p$. This induces: \begin{defn} \label{IdeleNorm}\hypertarget{IdeleNorm}{} The \emph{idele norm} \begin{displaymath} {\vert -\vert} \colon \mathbb{I}_{\mathbb{Q}} \longrightarrow \mathbb{C}^\times \end{displaymath} is the function given by \begin{displaymath} {\vert \alpha\vert} \coloneqq \underset{v}{\prod} {\vert \alpha_v\vert}_v \,. \end{displaymath} \end{defn} Notice that by construction there is a [[diagonal]] map $\mathbb{Q}^\times \to \mathbb{I}_{\mathbb{Q}}$. \begin{prop} \label{ProductFormula}\hypertarget{ProductFormula}{} \textbf{(product formula)} The idele norm, def. \ref{IdeleNorm}, is trivial on the diagonal of $\mathbb{Q}^\times$ inside the ideles, in that \begin{displaymath} (\alpha \in \mathbb{Q}^\times \to \mathbb{I}_{\mathbb{Q}}) \;\Rightarrow\; {\vert \alpha\vert} \coloneqq \underset{v}{\prod} {\vert \alpha_v\vert}_v = 1 \,. \end{displaymath} \end{prop} \begin{remark} \label{}\hypertarget{}{} The product formula, prop. \ref{ProductFormula}, says that the idele norm descends to the idele class group, def. \ref{IdeleClassGroup}. \end{remark} (e.g. \hyperlink{Garrett11}{Garrett 11, section 1}) \hypertarget{StrongApproximationTheorem}{}\subsubsection*{{Strong approximation theorem for the idele class group}}\label{StrongApproximationTheorem} \begin{prop} \label{StrongApproximationTheorem}\hypertarget{StrongApproximationTheorem}{} \textbf{(strong approximation form ideles)} The idele class group, def. \ref{IdeleClassGroup}, may be expressed as \begin{displaymath} \mathbb{Q}^\times \backslash \mathbb{A}_{\mathbb{Q}}^\times \simeq (0,\infty) \times \underset{p}{\prod} \mathbb{Z}_p^\times \,. \end{displaymath} \end{prop} (e.g. \hyperlink{GoldfeldHundley11}{Goldfeld-Hundley 11, prop. 1.4.5 and below (2.2.7)}) This implies that the [[ring of adeles]] may be decomposed into a rational and an idele class factor as: \begin{displaymath} \begin{aligned} \mathbb{A}_{\mathbb{Q}}^\times & \simeq \mathbb{Q}^\times \times (\mathbb{Q}^\times \backslash \mathbb{A}_{\mathbb{Q}}^\times) \\ & \coloneqq \underset{n \in \mathbb{Q}^\times}{\cup} n \cdot (\mathbb{Q}^\times \backslash \mathbb{A}_{\mathbb{Q}}^\times) \end{aligned} \,. \end{displaymath} (e.g. \hyperlink{GoldfeldHundley11}{Goldfeld-Hundley 11, prop. 1.4.6 and below (2.2.7)}) This decomposition is crucial in the discussion of the [[Riemann zeta function]] (see there) as an [[adelic integral]]. \hypertarget{automorphic_forms_and_relation_to_dirichlet_characters}{}\subsubsection*{{Automorphic forms and Relation to Dirichlet characters}}\label{automorphic_forms_and_relation_to_dirichlet_characters} The [[automorphic forms]] of the idele group are essentially [[Dirichlet characters]] in disguise (\hyperlink{GoldfeldHundley11}{Goldfeld-Hundley 11, below def. 2.1.4}) \hypertarget{FunctionFieldAnalogy}{}\subsubsection*{{Function field analogy}}\label{FunctionFieldAnalogy} Via the [[function field analogy]] one may understand any [[number field]] or [[function field]] $F$ as being the [[rational functions]] on an [[arithmetic curve]] $\Sigma$. Under this identification the [[ring of adeles]] $\mathbb{A}_F$ of $F$ has the interpretation of being the [[ring of functions]] on all punctured [[formal disks]] around all points of $\Sigma$, such that only finitely many of them do not extend to the given point. (\hyperlink{Frenkel05}{Frenkel 05, section 3.2}). This means for instance that the [[general linear group]] $GL_n(\mathbb{A}_F)$ with [[coefficients]] in the [[ring of adeles]] has the interpretation as being the [[Cech cohomology|Cech cocycles]] for [[algebraic vector bundles]] of [[rank]] $n$ on an [[algebraic curve]] with respect to any [[cover]] of that curve by the complement of a finite number of points together with the [[formal disks]] around these points. Here for $n = 1$ then $GL_1(\mathbb{A}_F)$ is the group of ideles. This is part of a standard construction of the [[moduli stack of bundles]] on algebraic curves, see at \emph{\href{moduli+space+of+bundles#OverCurvesAndTheLanglandsCorrespondence}{Moduli space of bundles and the Langlands correspondence}}. [[!include function field analogy -- table]] \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[differential cohesion and idelic structure]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} Basics are recalled in \begin{itemize}% \item \emph{Adeles} \href{http://wiki.epfl.ch/gant/documents/lecture2-cib2011.pdf}{pdf} \item Pete Clark, \emph{Adeles and Ideles} (\href{http://math.uga.edu/~pete/8410Chapter6.pdf}{pdf}) \item Erwin Dassen , \emph{Adeles \& Ideles} (\href{http://www.math.leidenuniv.nl/~astolk/monday/notes/dassen-adeles-ideles.pdf}{pdf}) \item Tom Weston, \emph{The idelic approach to number theory} (\href{http://www.math.umass.edu/~weston/oldpapers/idele.pdf}{pdf}) \item [[Dorian Goldfeld]], [[Joseph Hundley]], chapter 2 of \emph{Automorphic representations and L-functions for the general linear group}, Cambridge Studies in Advanced Mathematics 129, 2011 (\href{https://www.maths.nottingham.ac.uk/personal/ibf/text/gl2.pdf}{pdf}) \item [[Paul Garrett]], \emph{Iwasawa-Tate on $\zeta$-functions and L-functions}, 2011 (\href{http://www-users.math.umn.edu/~garrett/m/mfms/notes_c/Iwasawa-Tate.pdf}{pdf} [[!redirects Poisson formula]] \end{itemize} Discussion in the context of the [[geometric Langlands correspondence]] is in \begin{itemize}% \item [[Edward Frenkel]], section 3.2 of \emph{Lectures on the Langlands Program and Conformal Field Theory}, in \emph{Frontiers in number theory, physics, and geometry II}, Springer Berlin Heidelberg, 2007. 387-533. (\href{http://arxiv.org/abs/hep-th/0512172}{arXiv:hep-th/0512172}) \end{itemize} [[!redirects idele]] [[!redirects ideles]] [[!redirects idele class group]] [[!redirects idele class groups]] [[!redirects idele group]] [[!redirects idele groups]] [[!redirects idèle group]] [[!redirects group of idèles]] \end{document}