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Signed algebraic periods and a finite integral lattice

Proved
MTT.periods_exist

by davidloeffler · Sep 5, 2026 · Mathlib 0df444a (Lean v4.33.1)

modular-formsnumber-theoryp-adic-l-functions

Every normalized algebraic cuspidal Hecke eigenform of positive level and weight k ≥ 2 has two nonzero complex periods. Dividing each signed modular integral by the corresponding period gives algebraic values; their integral span, for all rational cusps and degrees 0 through k−2, is finitely generated. The signed projection includes one half and reflection of the polynomial.

Preamble
import Definitions.Def_MTT_Measures

set_option autoImplicit false
noncomputable section
open scoped BigOperators
Formal statement
theorem MTT.periods_exist
    {N k : ℕ} (hN : 0 < N) (hk : 2 ≤ k)
    (ι : MTT.Qbar →+* ℂ) (f : MTT.Eigenform N k ι) :
    Nonempty (MTT.Periods k ι f.form) := by sorry
Source
Mazur–Tate–Teitelbaum, On p-adic analogues of the conjectures of Birch and Swinnerton-Dyer, Invent. Math. 84 (1986), https://doi.org/10.1007/BF01388731; Chapter I, §2 Proposition, pp. 6–7, together with Manin–Shimura period rationality; Shimura, On the periods of modular forms (1977), https://doi.org/10.1007/BF01391466. For the general-eigenform reduction see Williams, An introduction to p-adic L-functions II, Proposition 11.21, https://warwick.ac.uk/fac/sci/maths/people/staff/cwilliams/lecturenotes/lecture_notes_part_ii.pdf. This milestone combines rationality and finite generation, rather than attributing both to a single proposition.
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What the Lean code literally says, in plain math · GPT-6 (Codex)

For every pair of natural numbers N,kN,kN,k with N>0N>0N>0 and k≥2k\ge 2k≥2, every unital ring homomorphism ι:Q‾→C\iota:\overline{\mathbb Q}\to\mathbb Cι:Q​→C from the algebraic closure of Q\mathbb QQ, and every collection consisting of a weight-kkk cusp form fff on Γ1(N)\Gamma_1(N)Γ1​(N), a Dirichlet character ε:Z/NZ→Q‾\varepsilon:\mathbb Z/N\mathbb Z\to\overline{\mathbb Q}ε:Z/NZ→Q​ (extended by zero on nonunits), and coefficients an∈Q‾a_n\in\overline{\mathbb Q}an​∈Q​ for all n∈Nn\in\mathbb Nn∈N, assume that the coefficient of degree nnn of the width-one qqq-expansion of fff equals ι(an)\iota(a_n)ι(an​) for every nnn, that a1=1a_1=1a1​=1, that for every γ=(abcd)∈Γ0(N)\gamma=\left(\begin{smallmatrix}a&b\\c&d\end{smallmatrix}\right)\in\Gamma_0(N)γ=(ac​bd​)∈Γ0​(N) and zzz in the complex upper half-plane one has f(γz)=ι(ε(d))(cz+d)kf(z)f(\gamma z)=\iota(\varepsilon(d))(cz+d)^k f(z)f(γz)=ι(ε(d))(cz+d)kf(z), and that for every prime natural number ℓ\ellℓ and every such zzz one has ℓ−1∑b=0ℓ−1f((z+b)/ℓ)+ι(ε(ℓ))ℓk−1f(ℓz)=ι(aℓ)f(z)\ell^{-1}\sum_{b=0}^{\ell-1}f((z+b)/\ell)+\iota(\varepsilon(\ell))\ell^{k-1}f(\ell z)=\iota(a_\ell)f(z)ℓ−1∑b=0ℓ−1​f((z+b)/ℓ)+ι(ε(ℓ))ℓk−1f(ℓz)=ι(aℓ​)f(z). Here Γ0(N)\Gamma_0(N)Γ0​(N) consists of determinant-one integer matrices with c≡0(modN)c\equiv0\pmod Nc≡0(modN), and Γ1(N)\Gamma_1(N)Γ1​(N) additionally has a≡d≡1(modN)a\equiv d\equiv1\pmod Na≡d≡1(modN), acting on the upper half-plane by fractional linear transformations. Then there exist two nonzero complex numbers Ω+,Ω−\Omega_+,\Omega_-Ω+​,Ω−​ and a function v:{+,−}×N×Q→Q‾v:\{+,-\}\times\mathbb N\times\mathbb Q\to\overline{\mathbb Q}v:{+,−}×N×Q→Q​ such that for both signs sss, every integer jjj with 0≤j≤k−20\le j\le k-20≤j≤k−2, and every rational rrr, ι(v(s,j,r))=(Ij(r)+σs(−1)jIj(−r))/(2Ωs)\iota(v(s,j,r))=(I_j(r)+\sigma_s(-1)^jI_j(-r))/(2\Omega_s)ι(v(s,j,r))=(Ij​(r)+σs​(−1)jIj​(−r))/(2Ωs​), where σ+=1\sigma_+=1σ+​=1, σ−=−1\sigma_-=-1σ−​=−1, and Ij(r)=2π∫(0,∞)f(r+it)(r+it)j dtI_j(r)=2\pi\int_{(0,\infty)}f(r+it)(r+it)^j\,dtIj​(r)=2π∫(0,∞)​f(r+it)(r+it)jdt is the complex Bochner integral with respect to real Lebesgue measure; moreover, the additive subgroup of Q‾\overline{\mathbb Q}Q​ consisting of all finite integer linear combinations of the values v(s,j,r)v(s,j,r)v(s,j,r) with s∈{+,−}s\in\{+,-\}s∈{+,−}, 0≤j≤k−20\le j\le k-20≤j≤k−2, and r∈Qr\in\mathbb Qr∈Q is finitely generated as a Z\mathbb ZZ-module. The function vvv is defined also at every j>k−2j>k-2j>k−2, but those values satisfy no comparison or finite-generation requirement. The statement includes N=1N=1N=1, k=2k=2k=2 (when only j=0j=0j=0 is constrained), and r=0r=0r=0; at r=0r=0r=0 the signed numerator is zero whenever σs(−1)j=−1\sigma_s(-1)^j=-1σs​(−1)j=−1. It excludes N=0N=0N=0 and k<2k<2k<2, imposes no uniqueness of the periods or values, and has no separate integrability hypothesis: the integrals are the total Bochner integrals, whose value is zero for a nonintegrable integrand.

Human review
  • Endorsed by Shuze Chen · Sep 5, 2026

  • Endorsed by davidloeffler · Sep 5, 2026

    Confirmed by the mission captain (proposal self-audit).

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