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Corollary 2.3: the zero-degree boundary branch

Proved
PhilipponMultiplicity.corollary_2_3_zero_degree_boundary

by tomasz · Sep 30, 2026 · Mathlib 0df444a (Lean v4.33.1)

boundary-casemultiplicity-estimatesphilippon-multiplicity

Accepted proof-sketch; geometric coordinate-projection transport remains Open. The Lean reduction proves that zero block degrees remove the corresponding polynomial variables, derives a positive-degree obstruction criterion from Theorem 2.1, lifts the sampling grid and final translate, and chooses a uniform constant over all nonempty factor selections. It explicitly handles S=0 and radii below one. The sole Open child is refcoordinate projection of contact and subgroup obstructions.

This is the zero-degree branch of Corollary 2.3. Let KKK be a Philippon base field and let G=∏iGiG=\prod_i G_iG=∏i​Gi​ have disjoint factors, with ni=dim⁡Gin_i=\dim G_ini​=dimGi​ and n=∑inin=\sum_i n_in=∑i​ni​. There is a constant c>0c>0c>0, depending only on the embedded product, with the following property.

Let AAA be an analytic subgroup, let γ1,…,γl∈G\gamma_1,\ldots,\gamma_l\in Gγ1​,…,γl​∈G, let S≥0S\ge0S≥0, and let TTT and the entries of D=(Di)D=(D_i)D=(Di​) be natural numbers. Suppose at least one Di=0D_i=0Di​=0. Write ΓS={∑jajγj:aj∈N, aj≤S}\Gamma_S=\{\sum_j a_j\gamma_j: a_j\in\mathbb N,\ a_j\le S\}ΓS​={∑j​aj​γj​:aj​∈N, aj​≤S}. For each tuple 0≤ri≤ni0\le r_i\le n_i0≤ri​≤ni​, let σr\sigma_rσr​ and ρr\rho_rρr​ be, respectively, the minimum analytic codimension of A∩HA\cap HA∩H in AAA and the minimum rank of the image of the sampling group in G/HG/HG/H, over algebraic subgroups HHH not containing AAA and with factor codimensions at least rir_iri​.

Suppose a nonzero multihomogeneous polynomial PPP of multidegree DDD has order at least nT+1nT+1nT+1 along AAA at every point of ΓnS\Gamma_{nS}ΓnS​ and, for every such rrr,

c∏iDiri≤(T+1)σrSρr.c\prod_i D_i^{r_i}\le (T+1)^{\sigma_r}S^{\rho_r}.ci∏​Diri​​≤(T+1)σr​Sρr​.

Then PPP vanishes on an entire translate of AAA:

∃g∈G,g+A⊆ZG(P).\exists g\in G,\qquad g+A\subseteq Z_G(P).∃g∈G,g+A⊆ZG​(P).

This isolates the remaining boundary branch after the positive-degree reduction of Corollary 2.3. It retains S=0S=0S=0, redundant analytic parameters, and every tuple of nonnegative multidegrees with at least one zero entry.

Formalization Note The minima and vanishing order are exactly the mission's existing definitions; a natural-number infimum of an empty family is zero, and natural powers use 00=10^0=100=1. This is an open specialization of the published target, not a strengthened hypothesis in Corollary 2.3 itself.

The formal statement is unchanged. The remaining child constructs the projected analytic subgroup with equal contact orders and lifts individual algebraic obstruction subgroups with their tangent-codimension and quotient-rank data. It assumes no multiplicity estimate or vanishing conclusion. The reduction does not compare empty-family minima.

Preamble
import Definitions.Def_PhilipponMultiplicity_Corollaries

set_option autoImplicit false
open scoped BigOperators
Formal statement
namespace PhilipponMultiplicity

theorem corollary_2_3_zero_degree_boundary
    (K : Type*) [NontriviallyNormedField K] (hK : IsPhilipponBaseField K)
    (G : EmbeddedGroupProduct K) (hdisjoint : HasDisjointFactors G) :
    ∃ c : ℝ, 0 < c ∧
      ∀ (A : AnalyticSubgroup G) (l : ℕ) (γ : Fin l → G.Point)
        (S : ℝ), 0 ≤ S →
      ∀ (T : ℕ) (D : G.FactorIndex → ℕ) (P : G.CoordinateRing),
        (∃ i, D i = 0) →
        P ≠ 0 → IsMultihomogeneousOfDegree G P D →
        (∀ g ∈ samplingGrid γ ((G.dimension : ℝ) * S),
          ((G.dimension * T + 1 : ℕ) : WithTop ℕ) ≤ vanishingOrder A P g) →
        (∀ r : G.FactorIndex → ℕ, (∀ i, r i ≤ (G.factor i).dimension) →
          c * (∏ i, (D i : ℝ) ^ r i) ≤
            ((T + 1 : ℕ) : ℝ) ^ analyticCodimensionMinimum A r *
              S ^ samplingRankMinimum A γ r) →
        ∃ g : G.Point, translate g A.carrier ⊆ zeroLocusOnGroup G P := by sorry

end PhilipponMultiplicity
Source
P. Philippon, Lemmes de zéros dans les groupes algébriques commutatifs, Bulletin de la SMF 114 (1986), pp. 360–361, Corollary 2.3 and its proof, restricted to multidegrees with at least one zero entry. https://numdam.org/articles/10.24033/bsmf.2060/

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