U1(S)
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The Hecke operator T_v as an R-linear map from R-valued quaternionic weight 2 automorphic forms of level U_1(S).
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The (global) matrix element diag[α, 1].
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The (global) matrix element (unipotent t) * (diag α hα) = !![α, t; 0, 1].
Here t ∈ 𝒪ᵥ / α and we lift it arbitrarily to 𝒪ᵥ.
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The set of elements unipotent_mul_diag, that is, the elements of (D ⊗ 𝔸_F^∞)ˣ
which are (α t;0 1) at v and the identity elsewhere, as t runs through a set
of coset reps of 𝓞ᵥ / α. These will form a set of coset representatives for U1 diag U1.
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The double coset space U₁(S) diag(αᵥ,1) U₁(S) as a set of left cosets.
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The Hecke operator U_{v,α} associated to the matrix (α 0;0 1) at v,
considered as an R-linear map from R-valued quaternionic weight 2
automorphic forms of level U_1(S). Here α is a nonzero element of 𝓞ᵥ.
We do not demand the condition v ∈ S, the bad primes, but this operator
should only be used in this setting. See also T r v for the good primes.
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HeckeAlgebra F D r S R is the Hecke algebra associated to the weight 2
R-valued automorphic forms associated to the discriminant 1 totally definite
quaternion algebra D over the totally real field F, of level U₁(S) where S is
a finite set of nonzero primes v of 𝓞 F. To make sense of this definition we choose
a rigidification r, that is, a fixed 𝔸_F^∞-linear
isomorphism D ⊗[F] 𝔸_F^∞ = M₂(𝔸_F^∞), enabling us to define level structures and
Hecke operators Tᵥ and Uᵥ using 2x2 matrices.
Details: U₁(S) is the subgroup of (D ⊗[F] 𝔸_F^∞)ˣ associated, via r, to the
matrices which are in GL₂(𝓞ᵥ) for all v ∉ S and which are of the form
(a *; 0 a) mod v for all v ∈ S. The Hecke algebra is defined to be the
sub-R-algebra of the weight 2 forms of level U₁(S) generated by the following
two kinds of Hecke operators: first there are the operators
Tᵥ associated to the matrices (ϖᵥ 0; 0 1) for v ∉ S (here ϖᵥ ∈ 𝔸_F^∞ is a local
uniformiser supported at v). Second, there are the Hecke operators Uᵥ,ₐ
for v ∈ S and 0 ≠ αᵥ ∈ 𝓞ᵥ, associated the matries (αᵥ 0; 0 1).
These slightly nonstandard Hecke operators satisfy Uᵥ,ₛ * Uᵥ,ₜ = Uᵥ,ₛₜ
and in particular this Hecke algebra is commutative (Hecke operators supported
at distinct primes commute because the decomposition of the double cosets
into single cosets can be done purely locally).
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The Hecke operator Tᵥ as an element of the Hecke algebra.
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The Hecke operator Uᵥ,ₐ as an element of the Hecke algebra.