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Even-index Zeilberger–Zudilin integrals are integer linear forms in 111 and π\piπ

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PiIrrationality.ZZEven.linearForm

by moona3k · Oct 5, 2026 · Mathlib 0df444a (Lean v4.33.1)

diophantine-approximationnumber-theorypi

With RnR_nRn​, JnJ_nJn​, coefn\mathrm{coef}_ncoefn​, Φn\Phi_nΦn​ and MnM_nMn​ as in the definition file PiIrrationality_ZZEvenForms, for every n≥1n\ge1n≥1 there are integers UnU_nUn​ and VnV_nVn​ with

Mn Jn=Un+Vnπ,Vn=−Mn 16n coefn4.M_n\,J_n=U_n+V_n\pi,\qquad V_n=-\frac{M_n\,16^n\,\mathrm{coef}_n}{4}.Mn​Jn​=Un​+Vn​π,Vn​=−4Mn​16ncoefn​​.

This is the arithmetic of the Zeilberger–Zudilin construction at even index. The integrand has the even partial-fraction decomposition Rn=Pn+∑j=06ncj,n((5+t)−j−1+(5−t)−j−1)R_n=P_n+\sum_{j=0}^{6n}c_{j,n}\bigl((5+t)^{-j-1}+(5-t)^{-j-1}\bigr)Rn​=Pn​+∑j=06n​cj,n​((5+t)−j−1+(5−t)−j−1) with Pn∈Z[t2]P_n\in\mathbb{Z}[t^2]Pn​∈Z[t2]. Only the j=0j=0j=0 term produces π\piπ, through ∫(15+t+15−t) dt=πi/2\int(\tfrac1{5+t}+\tfrac1{5-t})\,dt=\pi i/2∫(5+t1​+5−t1​)dt=πi/2. The multiplier Mn=24−5nlcm⁡(1,…,8n)/ΦnM_n=2^{4-5n}\operatorname{lcm}(1,\dots,8n)/\Phi_nMn​=24−5nlcm(1,…,8n)/Φn​ clears all denominators: the 222-adic and 555-adic valuations of the Laurent coefficients give the power of 222, and the deleted primes in Φn\Phi_nΦn​ divide every relevant coefficient. The π\piπ-coefficient is −Mnc0,n/2-M_nc_{0,n}/2−Mn​c0,n​/2. The residue satisfies c0,n=16n coefn/2c_{0,n}=16^n\,\mathrm{coef}_n/2c0,n​=16ncoefn​/2, by the substitution z=(t+5)/(t−5)z=(t+5)/(t-5)z=(t+5)/(t−5), which turns Rn(t) dtR_n(t)\,dtRn​(t)dt into 1216nz−6n−1S(z)n dz\tfrac12 16^n z^{-6n-1}S(z)^n\,dz21​16nz−6n−1S(z)ndz.

Preamble
import Definitions.Def_PiIrrationality_ZZEvenForms
import Mathlib.Analysis.SpecialFunctions.Trigonometric.Basic
Formal statement
theorem PiIrrationality.ZZEven.linearForm (n : ℕ) (hn : 1 ≤ n) :
    ∃ U V : ℤ,
      (PiIrrationality.ZZEven.M n : ℂ) * PiIrrationality.ZZEven.J n =
          (U : ℂ) + (V : ℂ) * (Real.pi : ℂ) ∧
        (V : ℝ) = -(PiIrrationality.ZZEven.M n * 16 ^ n *
          (PiIrrationality.ZZEven.coef n : ℝ) / 4) := by
  sorry
Source
Y. Bai, The irrationality measure of π is at most 7.101862832357, arXiv:2609.11276 (v2, 11 Sep 2026), Section 2: Lemmas 2.1–2.6 and Proposition 2.7 (with (a,b,c)=(2,4,6), so h=5 and d0=8), and equation (4.5); D. Zeilberger and W. Zudilin, The irrationality measure of π is at most 7.103205334137…, Moscow J. Combin. Number Theory 9 (2020), no. 4, 407–419, arXiv:1912.06345, Lemmas 1–6 and Proposition 1 at index 2n.

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