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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.

3 votes

On the local factor of Rankin-Selberg L-functions

If we write $$L_p( f\times g, s) =\prod_{i=1}^4 \left(1 - \frac{m_i}{p^s} \right)^{-1}$$ for $p\nmid N$ and $L_p ( f\times g, s) =\prod_{i=1}^2 \left(1 - \frac{m_i}{p^s} \right)^{-1}$ then the Rankin …
Will Sawin's user avatar
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6 votes
Accepted

Adelization for any classical arithmetic subgroup

For a subgroup to have a meaningful lift to the adeles, it is necessary and sufficient for the subgroup to be a congruence subgroup in the sense that for some $N$, the subgroup contains all elements c …
Will Sawin's user avatar
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3 votes

How to relate Rankin triple L-function to its Dirichlet series

It's easier to give an elementary formula for this sum directly than in terms of the sum you give, though one can give a formula in terms of this sum by taking a ratio of Euler factors. Let $q$ be a p …
Will Sawin's user avatar
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11 votes
Accepted

Equivalence between Ramanujan and Selberg conjectures

Let $f$ be an automorphic form corresponding to an automorphic representation $\pi =\otimes_v \pi_v$ of $GL_2(\mathbb A_{\mathbb Q})$. For an unramified prime $p$, the following are equivalent (for …
Will Sawin's user avatar
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15 votes
Accepted

How can I see the relation between shtukas and the Langlands conjecture?

I think shtukas are best understood ahistorically. I would start with the modular curves, but specifically with the (geometric) Eichler-Shimura relation. This says that the Hecke operator at $p$, view …
Will Sawin's user avatar
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16 votes
Accepted

The space of cusp forms for $\mathrm{GL}_2$ over ${\mathbf{F}}_q(T)$

If $D$ has degree $\leq 3$ there won't be any cusp forms. By the Langlands correspondence these correspond to irreducible Galois representations into $GL_2$, unramified away from a degree $3$ divisor, …
Will Sawin's user avatar
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8 votes

Why is the Langlands dual group always taken over $\mathbb{C}$?

The Satake isomorphism gives a relationship between the convolution algebra of $F$-valued functions on $G (\mathcal O) \backslash G(K) / G(\mathcal O)$ and the ring of conjugacy-invariant polynomial f …
Will Sawin's user avatar
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6 votes

About different cohomology theories used to study Shimura varieties

For Eichler-Shimura, I am not a historian, but I have a guess. I think the first papers of Eichler and Shimura handle only the correspondence between modular forms of weight 2 and the Jacobians of (co …
Will Sawin's user avatar
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8 votes

Relation between Fourier coefficients and Satake parameters

There is no coincidence, this is the Weyl character formula for the representation $\operatorname{Sym}^k$ of $GL_3$. The reason that the Langlands dual group comes up is, unsurprisingly, the Satake is …
Desiderius Severus's user avatar
13 votes

Estimating volumes in the trace formula

Up to a normalisation factor for the measure you fixed on $G_\gamma(\mathbb A)$, this is the Tamagawa number of $G_\gamma$. There is indeed a formula for this. It is $$ \frac{ \left| \pi_0\left( Z(\ …
Will Sawin's user avatar
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16 votes
Accepted

What is the matter with Hecke operators?

If $K$ is a local field and $\mathcal O$ is its ring of integers, we say an irreducible representation of $G(K)$ is unramified if it contains a vector invariant under $G(\mathcal O)$. It is known that …
Desiderius Severus's user avatar
9 votes
Accepted

Global Langlands function fields

The abstract of V. Lafforgue's paper https://arxiv.org/abs/1404.6416 says For any reductive group G over a global function field, we use the cohomology of G-shtukas with multiple modifications and …
Will Sawin's user avatar
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10 votes

Elliptic curves and supercuspidal representations of conductor $p^2$

Because $p \geq 5$, the ramification of the Galois representation is tame, hence the action of the inertia group on that Galois representation factors through a cyclic group. For the exact same define …
Will Sawin's user avatar
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4 votes
Accepted

Is the universal elliptic curve $\overline M_{1,2}$ a toric stack?

The fibers aren't rational curves when viewed as stacks, because they have four points with extra automorphisms. I think this will be problematic. The universal family of elliptic curves is the quoti …
Will Sawin's user avatar
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1 vote

Explicit examples of algebraic Hecke characters with infinite image?

Let $n$ be the maximal order of an element of the class group. Take an ideal, take the $n$th power, take a generator, square it, divide by its conjugate.
Will Sawin's user avatar
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