Vertical/nonvertical irreducible pair meets in at most points
ProvedPachDeZeeuw.Algebraic.zeroCurry_nonvertical_pair_intersection_boundbezoutintersection-boundspach-de-zeeuw-bezoutvertical-lines
Let and be irreducible bivariate real polynomials (, ) that are not associated (), of total degree at most and (, ). Suppose is vertical, , while genuinely depends on the first variable, . Then the intersection is finite with the clean Bezout product bound:
This handles the mixed vertical/nonvertical irreducible case: the vertical curve contributes at most lines, and on each line restricts to a nonzero univariate polynomial of degree at most . It supplies the sharp 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
Confirmed by the mission captain (proposal self-audit).