Notes
A representation of \(G\) of finite dimension \(n\) consists in attaching to each element \(g\) in \(G\) an invertible square matrix \(M_{g}\) of size \(n\) that satisfies \(M_{g\cdot g'}=M_{g} M_{g'}\) for all \(g\) and \(g'\) in \(G\).
An automorphism \(\sigma \) of a field \(L\) is a bijective application \(\sigma \colon L \to L\) such that \(\sigma (a+b)=\sigma (a)+ \sigma (b)\) and \(\sigma (ab)=\sigma (a)\sigma (b)\), for all \(a\) and \(b\) in \(L\). The set of automorphisms of \(L\), provided with composition law of applications, forms a group; the automorphisms of \(L\) that leave unchanged the elements of \(K\) is a subgroup of the latter.
A field extension \(L/K\) is called algebraic if every element of \(L\) is algebraic over \(K\), i.e., if every element of \(L\) is a root of some non-zero polynomial with coefficients in \(K\). A field \(L\) is called algebraically closed if it contains a root for every non-constant polynomial in \(L[X]\), the ring of polynomials in the variable \(X\) with coefficients in \(L\). An algebraic closure of \(K\) is an algebraic extension of \(K\) that is algebraically closed.
The group \(\mathrm{GL}_{n}({\mathbb{A}}_{{\mathbb{Q}}})\) consists in invertible \(n\times n\) matrices with entries in \({\mathbb{A}}_{{\mathbb{Q}}}\).
More precisely, these are representations of a certain group \({\mathcal{L}}_{{\mathbb{Q}}}\), called the Langlands group of ℚ, which is close to the group \(\varGamma_{{\mathbb{Q}}}\). However, the group \({\mathcal{L}} _{{\mathbb{Q}}}\) is complicated and its existence is still conjectural in general, even, in the case when \(n=2\), see Sect. 10.
I.e., \(K\) is either a finite extension of ℚ or a finite extension of \({\mathbb{F}}_{q}(t)\), where \({{\mathbb{F}} _{q}}\) is a finite field with \(q\) elements and \(t\) is an indeterminate. In the first case, \(K\) is called a number field, otherwise it is called a global function field.
See Sect. 7.
Among modular forms, the cusp forms are distinguished by the vanishing in their Fourier series expansion \(\sum_{n\ge 0}a_{n} q^{n}\) of the constant coefficient \(a_{0}\).
See Sect. 3.
See Sect. 3.
Any inner form of \(\mathrm{GL}_{n}(F)\) is given by the group of invertible \(m\times m\) matrices \(\mathrm{GL}_{m}(D)\), with entries in a division algebra \(D\) with center \(F\) and such \(\dim_{F} (D) = d^{2}\), where \(dm=n\).
A tempered representation of a linear semisimple Lie group \(G\) is a representation that has a basis whose matrix coefficients lie in the \(L^{p}\) space \(L^{2+\varepsilon }(G)\), for any \(\varepsilon > 0\). This condition is slightly weaker than the condition that the matrix coefficients are square-integrable, in other words lie in \(L^{2}(G)\), which would be the definition of a discrete series representation.
See Sect. 3.
See Sect. 5.
I.e., an \(L\) -homomorphism as defined in 5.1.
I.e., if any irreducible polynomial with coefficients in \(F\) and a root in the extension actually splits into a product of linear factors as polynomial with coefficients in the extension.
A Hausdorff topological space is a topological space in which distinct points have disjoint neighborhoods.
This means that it is possible to consider the \(F\)-points of \({\mathbf{G}}\).
It is a direct product if \(G\) is \(F\)-split.
A distribution on \(G\) is defined to be a linear functional on \(C_{{\mathrm{c}}}^{\infty }(G)\). A distribution \(D\) on \(G\) is said to be \(G\)-invariant (or invariant) if \(D(fg) = D(f)\) for all \(g\in G\).
Recall that two elements \(\gamma \) and \(\gamma '\) in \(G={\mathbf{G}}(F)\) are said to be conjugate if there is \(g\in G\) such that \(\gamma '=g\gamma g ^{-1}\). A conjugacy class \(C\) is the set of conjugates of an element in \(G\).
An element \(\gamma \) of \({\mathbf{G}}\) is called semisimple if it is contained in a torus, it is called regular if the identity component of its centralizer is a maximal torus, and strongly regular if its centralizer itself is a maximal torus. A regular semisimple element \(\gamma \) of \(G\) is called elliptic if its centralizer in \(G\) is compact modulo the center of \(G\). In \(G=\mathrm{GL}_{n}(F)\), the semi-simple conjugacy classes \((\gamma )\) are classified by their characteristic polynomial \(P_{\gamma }(X)\in F[X]\); the element \(\gamma \) is regular if and only if the roots of \(P_{\gamma }(X)\) are simple, and \(\gamma \) is elliptic if the quotient \(F[X]/P_{\gamma }(X)F[X]\) is a field of degree \(n\) over \(F\).
A discrete series representation of a locally compact topological group \(G\) is an irreducible unitary representation of \(G\) that is a subrepresentation of the left regular representation of \(G\) on \(L^{^{2}}(G)\).
Note that \(f_{\pi }\) is highly non-unique. However as regards invariant harmonic analysis this plays no role.
This reflects the fact that we consider simultaneously all the inner twists of a given group \(G\).
The set of (equivalence classes of) inner twists of \({\mathbf{G}}\) is parametrized by the Galois cohomology group \(H^{1}(F,{\mathbf{G}}^{*} _{\mathrm{ad}})\); we parametrize the quasi-split twist of \({\mathbf{G}}\) by \(\zeta =1\).
The space \(L^{2}({\mathbf{G}}(K) \backslash {\mathbf{G}}({\mathbb{A}}_{K}), \omega )\) is an Hilbert space under the inner product \((f_{1},f_{2})\mapsto \int_{{\mathbf{Z}}({\mathbb{A}}_{K}){\mathbf{G}}(K)\setminus {\mathbf{G}}( {\mathbb{A}}_{K})} f_{1}(g)\overline{f_{2}(g)}dg\), and it admits a unitary action of \({\mathbf{G}}({\mathbb{A}}_{K})\) by right translations: \({(g' \cdot f)}(g):= f(gg')\) for \(g'\in {\mathbf{G}}( {\mathbb{A}}_{K})\).
The modular group is the group of transformations \(\varGamma (1):=\mathrm{GL}_{2}({\mathbb{Z}})^{+}\) considered in Sect. 7. It is isomorphic to the projective special linear group \(\mathrm{PSL}_{2}({\mathbb{Z}})\) of \(2 \times 2\) matrices with coefficients in ℤ and unit determinant.
See for instance [25, §1.6].
We have \(G=\mathrm{Res}_{L/K}(\mathrm{GL}_{n})\), where \(\mathrm{Res}_{L/K}\) is the Weil restriction of scalars defined in [99].
An \(E\)-Banach space is a topological \(E\)-vector space which is complete with respect to the topology given by a norm; when \(E={\overline{\mathbb{Q}}_{\ell }}\), it is called an \(\ell \)-adic Banach space.
A Größen-character—or Hecke character—of \(K\) is a family of morphisms \(\chi =(\chi _{x})\) indexed by the places of \(K\), with \(\chi_{x}\colon K_{x}^{ \times }\), satisfying \(\prod_{x\in |C|}\chi_{x}(a)=1\) for each \(a\in K^{\times }\).
A function is called smooth if it is invariant by an open compact subgroup.
It is the group of isomorphism classes of invertible sheaves (or line bundles) on \(C\), with the group operation being tensor product.
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Aubert, AM. Around the Langlands Program. Jahresber. Dtsch. Math. Ver. 120, 3–40 (2018). https://doi.org/10.1365/s13291-017-0171-8
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DOI: https://doi.org/10.1365/s13291-017-0171-8