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Vertical/nonvertical irreducible pair meets in at most d1d2d_1 d_2d1​d2​ points

Proved
PachDeZeeuw.Algebraic.zeroCurry_nonvertical_pair_intersection_bound

by mysticflounder · Sep 17, 2026 · Mathlib 0df444a (Lean v4.33.1)

bezoutintersection-boundspach-de-zeeuw-bezoutvertical-lines

Let hhh and kkk be irreducible bivariate real polynomials (hhh_hhh​, hkh_khk​) that are not associated (hnoth_{not}hnot​), of total degree at most d1d_1d1​ and d2d_2d2​ (hdegh_{deg}hdeg​, kdegk_{deg}kdeg​). Suppose hhh is vertical, hdeg0:(Curry0 h).natDegree=0h_{deg0} : (\mathrm{Curry0}\,h).\mathrm{natDegree} = 0hdeg0​:(Curry0h).natDegree=0, while kkk genuinely depends on the first variable, kpos:0<(Curry0 k).natDegreek_{pos} : 0 < (\mathrm{Curry0}\,k).\mathrm{natDegree}kpos​:0<(Curry0k).natDegree. Then the intersection is finite with the clean Bezout product bound:

(PlaneCurveZeroSet h∩PlaneCurveZeroSet k).Finite∧(PlaneCurveZeroSet h∩PlaneCurveZeroSet k).ncard≤d1⋅d2.{(\mathrm{PlaneCurveZeroSet}\,h \cap \mathrm{PlaneCurveZeroSet}\,k).\mathrm{Finite} \land (\mathrm{PlaneCurveZeroSet}\,h \cap \mathrm{PlaneCurveZeroSet}\,k).\mathrm{ncard} \leq d_1 \cdot d_2.}(PlaneCurveZeroSeth∩PlaneCurveZeroSetk).Finite∧(PlaneCurveZeroSeth∩PlaneCurveZeroSetk).ncard≤d1​⋅d2​.

This handles the mixed vertical/nonvertical irreducible case: the vertical curve hhh contributes at most d1d_1d1​ lines, and on each line kkk restricts to a nonzero univariate polynomial of degree at most d2d_2d2​. It supplies the sharp d1d2d_1 d_2d1​d2​ estimate for this case in the case split assembling the full irreducible pair-intersection bound.

Preamble
import Mathlib
import Definitions.Def_PdzPrelim

open EuclideanGeometry
open scoped Topology
open PachDeZeeuw.Algebraic
open PachDeZeeuw.Algebraic.PlaneCurve
Formal statement
theorem PachDeZeeuw.Algebraic.zeroCurry_nonvertical_pair_intersection_bound (h k : MvPolynomial (Fin 2) ℝ)
    {d₁ d₂ : ℕ}
    (hh : Irreducible h) (hk : Irreducible k)
    (hdeg : h.totalDegree ≤ d₁)
    (kdeg : k.totalDegree ≤ d₂)
    (hnot : ¬ Associated h k)
    (hdeg0 : (Curry0 h).natDegree = 0)
    (kpos : 0 < (Curry0 k).natDegree) :
    (PlaneCurveZeroSet h ∩ PlaneCurveZeroSet k).Finite ∧
      (PlaneCurveZeroSet h ∩ PlaneCurveZeroSet k).ncard ≤ d₁ * d₂ := by sorry
Source
Standard algebra used in proving Bézout's inequality, Theorem 2.1 of Pach–de Zeeuw, Distinct distances on algebraic curves in the plane (arXiv:1308.0177), which cites Gibson, Elementary Geometry of Algebraic Curves, Lemma 14.4; this lemma is not stated in the paper; formalized in https://github.com/mysticflounder/lean-formalizations/blob/dd46c17a2a034d7bfa0df02e7f77834d35592864/lean/LeanFormalizations/PachDeZeeuw/AlgebraicPrelim.lean#L1240-L1268
Human review
  • Endorsed by Shuze Chen · Sep 17, 2026

  • Endorsed by mysticflounder · Sep 17, 2026

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

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