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An automorphic form is a well-behaved function from a topological group $G$ to the complex numbers (or complex vector space) which is invariant under the action of a discrete subgroup $\Gamma \subset G$ of the topological group. Automorphic forms are a generalization of the idea of periodic functions in Euclidean space to general topological groups.

9 votes
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Generalizations of Hamburger's Theorem

Hamburger's theorem has been generalized in various ways to automorphic $L$-functions (of arbitrary degree). Such generalizations are called "converse theorems", and they play a central role in the La …
GH from MO's user avatar
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6 votes
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Is "self-dual" equivalent to "dihedral" for Maass forms on $\mathrm{GL}(2)$?

Let $f$ be a Maass form on the upper half-plane with nebentypus $\chi$. It is known that \begin{align*} \text{$f$ is self-dual}&\qquad\Longleftrightarrow\qquad\text{$L(f\times f,s)$ has a pole at $s=1 …
GH from MO's user avatar
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3 votes

The lower bound for the automorphic $L$-function $L(s,\pi)$ at the edge of the critical stri...

This is not known. The best known lower bound is due to Brumley (2013), to be found in an Appendix to a paper by Lapid, which gives that $$|L(1+it,\pi)| \gg_{\pi_{\infty},\varepsilon} ( N (1+|t|) …
GH from MO's user avatar
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7 votes

A question on hybrid subconvexity for individual L-functions

There are several confusions in your post. 1. Automorphic forms for the group $\mathrm{SL}_2(\mathbb{Z})$ have level $q=1$ by definition. You probably wanted to talk about newforms for the Hecke congr …
GH from MO's user avatar
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5 votes
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'$\times$' or '$\otimes$' when writing $L$-functions?

The symbol $\times$ on the left-hand side is the Rankin-Selberg product. If $\pi$ and $\rho$ are automorphic representations of $\mathrm{GL}(m)$ and $\mathrm{GL}(n)$, respectively, then one can define …
GH from MO's user avatar
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2 votes
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Question on automorphic $L$-functions

Let us restrict to automorphic representations of $\mathrm{GL}_n$ over $\mathbb{Q}$ with arbitrary $n$ and unitary central character. If $\pi$ is an irreducible cuspidal representation, then $L(s,\pi) …
GH from MO's user avatar
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8 votes
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On the notion of cuspidality

To every local admissible representation $\pi_v$ of $\mathrm{GL}_n(k_v)$, there is a local $L$-function $L(s,\pi_v)$. For a global admissible representation $\pi=\otimes_v \pi_v$ of $\mathrm{GL}_n(\ma …
GH from MO's user avatar
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10 votes
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Are the L-functions of a normalized newform and the corresponding cuspidal representation eq...

$L(\pi,s)$ agrees with $L(f,s)$ if $f\in\pi$ is a newform, and this is even true for $\mathrm{GL}_n$. Of course, things are complicated by the fact that there are many ways to define $L(\pi,s)$ and $L …
GH from MO's user avatar
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11 votes
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Absolute convergence of Rankin–Selberg series

This is an elaboration of Lucia's comment. Let us consider the Dirichlet coefficients of $L(s,\pi\times\pi')$, $L(s,\pi\times\tilde\pi)$, $L(s,\pi'\times\tilde\pi')$ at a prime power $p^k$. Following …
LSpice's user avatar
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4 votes

On the precise form of $\mathrm{GL}(3)$ (and others) L-functions

Whether you use $A(1,n)$ or $A(n,1)$ is just a convention. If one sequence defines $L(s,\pi)$, then the other one defines the dual $L$-function $L(s,\tilde\pi)$. For example, we could define the Diric …
GH from MO's user avatar
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4 votes
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$\DeclareMathOperator\SL{SL}$Multiplicities of irreducible representations in discrete part ...

You are asking what is known about the dimension of weight $k$ holomorphic cusp forms for $\mathrm{SL}_2(\mathbb{Z})$, and the multiplicities of Laplace eigenvalues of weight $0$ and weight $1$ Maass …
GH from MO's user avatar
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4 votes

Is a reductive adelic group a Type I group?

Freitag and van Dijk proved that the adelic points of a reductive group over a global field is trace class (Theorem 2.3), while every trace class group is of type I (Theorem 1.7). So the answer is yes …
GH from MO's user avatar
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7 votes
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Asymptotic behaviour of $K$-Bessel function in transition range

For a published account of the corrected proof, see Section 10 in Blomer-Holowinsky: Bounding sup-norms of cusp forms of large level, Invent. Math. 179 (2010), 645-681. See especially pages 679-680, w …
GH from MO's user avatar
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5 votes
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A question related to newform and irreducible cuspidal representation of $\operatorname{GL}_n$

Newform theory for $\mathrm{GL}_n$ was originally developed over non-archimedean local fields (at least for $n\geq 3$). The local statements readily yield their global adelic counterparts, since a cus …
Peter Humphries's user avatar
3 votes
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Under Ramanujan conjecture, is primitivity equivalent to cuspidality and irreducibility?

Yes, and this is true without the Ramanujan conjecture (but see also Peter Humphries' comment below). If $\pi$ is not irreducible, say $\pi=\pi_1\oplus\pi_2$, then $L(s,\pi)=L(s,\pi_1)L(s,\pi_2)$. …
GH from MO's user avatar
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