The derivative of the endpoint radius correction is controlled by the strong wall

Proved

BianchiIX.exists_restartSectionRadiusTail_derivative_bound

by trppppp · Nov 6, 2026 · Mathlib 0df444a (Lean v4.33.1)

bianchi-ix-era-statisticsdynamical-systems

Write $\Phi_t x$ for the normalized vacuum Bianchi IX flow and $x_h(Z)$ for the constraint-one, negative-first-shear chart with primary wall $h$ and center coordinates $Z=(v,b,c)$ (second shear and the other two walls). At time $t$ , denote its primary wall by $n_t$ and center by $Z_t$ . Set $r(Z)=\sqrt{v^2+(b-c)^2}$ . For $u=\sqrt{1-h^2}$ define $A=3/(2+u)$ , $B=3\sqrt3 h/(2+u)^2$ , $H=9(1+u)/(2(2+u)^2)$ , $Q_h(v,w)=(Av+Bw)^2+H^2w^2$ and $F_h(Z)=\sqrt{Q_h(v,b-c)}/\sqrt3$ . There is an absolute $C\ge32$ . Use $h>0$ , $Ch\le1/4$ , $0<b_0\le1/16$ , $T\ge0$ , $Cb_0T\le1/8$ and $0\le t\le T$ . For each $c>0$ there is $K>0$ . When $\|Z\|\le b_0/2$ , $r(Z)\ge cb_0$ , let $R_t$ be the radius defined by the quadratic form at the current strong wall and center. Then $R_t-r(Z_t)$ is differentiable in $Z$ and $\|D(R_t-r(Z_t))\|\le K n_t$ .

Preamble

import Definitions.Def_BianchiIX_RadialTransportData
import Definitions.Def_BianchiIX_Dynamics_WallEvolution
import Mathlib.Analysis.Calculus.ContDiff.Operations
import Mathlib.Analysis.Calculus.MeanValue
import Mathlib.Analysis.Calculus.Deriv.Prod
import Mathlib.Analysis.Normed.Module.FiniteDimension
import Mathlib.Topology.Order.Compact
import Mathlib.Tactic.NormNum
import Mathlib.Analysis.Calculus.Deriv.Pow
import Mathlib.Analysis.Calculus.Deriv.Mul
import Mathlib.Analysis.Calculus.Deriv.Add
import Mathlib.Analysis.Real.Sqrt
import Mathlib.Tactic.Linarith
import Mathlib.Tactic.Ring
import Mathlib.Data.Fin.VecNotation
import Mathlib.Tactic.FinCases
import Mathlib.Tactic.FunProp
import Mathlib.Analysis.SpecialFunctions.Sqrt
import Mathlib.Analysis.Calculus.Deriv.Inv
import Mathlib.Tactic.FieldSimp
import Mathlib.Analysis.SpecialFunctions.Log.Deriv
import Mathlib.Analysis.SpecialFunctions.ExpDeriv
import Mathlib.Analysis.Calculus.ContDiff.Comp
import Mathlib.Analysis.Calculus.Deriv.Comp
import Mathlib.Analysis.SpecialFunctions.Pow.Deriv
import Mathlib.Analysis.SpecialFunctions.Integrals.Basic
import Mathlib.Analysis.ODE.ExistUnique
import Mathlib.Analysis.SpecialFunctions.Arsinh
import Mathlib.Tactic.LinearCombination
import Mathlib.Analysis.SpecialFunctions.Arcosh
import Mathlib.MeasureTheory.Integral.IntervalIntegral.FundThmCalculus
import Mathlib.Analysis.Calculus.InverseFunctionTheorem.Deriv
import Mathlib.Analysis.Calculus.Deriv.MeanValue
import Mathlib.Analysis.Calculus.FDeriv.Partial
import Mathlib.Analysis.SpecificLimits.Basic
import Mathlib.Analysis.SpecialFunctions.Pow.Real
import Mathlib.Analysis.ODE.Gronwall
import Mathlib.Topology.Order.IntermediateValue
import Mathlib.Analysis.Calculus.FDeriv.Symmetric
import Mathlib.Analysis.Calculus.ContDiff.Deriv
import Mathlib.Tactic.GCongr
import Mathlib.Tactic.Positivity

noncomputable section
open BianchiIX
open Set

Formal statement

theorem BianchiIX.exists_restartSectionRadiusTail_derivative_bound :
    ∃ C ≥ (32 : ℝ), ∀ c : ℝ, 0 < c → ∃ K : ℝ, 0 < K ∧
      ∀ (h b T : ℝ) (Z : RestartCenterState),
        0 < h → C * h ≤ 1 / 4 → (0 < b ∧ b ≤ 1 / 16) → 0 ≤ T → C * b * T ≤ 1 / 8 →
        ‖Z‖ ≤ b / 2 → c * b ≤ restartRadialNorm Z → ∀ t ∈ Icc (0 : ℝ) T,
          DifferentiableAt ℝ (restartSectionRadiusTail h t) Z ∧
          ‖fderiv ℝ (restartSectionRadiusTail h t) Z‖ ≤ K * restartSectionStrongWall h t Z := by sorry

Source

https://github.com/l46430640-del/ERA-STATISTICS-FORMAL/blob/d74c241819f896ab8d050b21ace80c8e90eabd9e/BianchiIX/Dynamics/RestartRadiusTailDerivative.lean#L55-L145

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