Primitive nonvertical coprime pair has finite explicitly bounded intersection
ProvedPachDeZeeuw.Algebraic.primitive_nonvertical_pair_intersection_boundbezoutintersection-boundspach-de-zeeuw-bezoutresultants
Let and be bivariate real polynomials of total degree at most and respectively (, ), with primitive curries (, ), both genuinely depending on the first variable (, likewise), and coprime curries (). Then their common zero set is finite with an explicit bound:
This is the workhorse resultant-based intersection estimate of the preliminaries: the resultant in the coefficient variable has degree at most , its roots index the nonempty fibers, and each fiber has at most points. Summing over components upgrades it to the finite Bezout bound for arbitrary (reducible) curves.
Preamble
import Mathlib import Definitions.Def_PdzBezout import Definitions.Def_PdzPrelim open EuclideanGeometry open scoped Topology open PachDeZeeuw.Algebraic open PachDeZeeuw.Algebraic.PlaneCurve
Formal statement
theorem PachDeZeeuw.Algebraic.primitive_nonvertical_pair_intersection_bound (p q : MvPolynomial (Fin 2) ℝ)
{d₁ d₂ : ℕ}
(hpdeg : p.totalDegree ≤ d₁)
(hqdeg : q.totalDegree ≤ d₂)
(hpprim : (Curry0 p).IsPrimitive)
(hqprim : (Curry0 q).IsPrimitive)
(hp0deg : 0 < (Curry0 p).natDegree)
(hq0deg : 0 < (Curry0 q).natDegree)
(hrel : IsRelPrime (Curry0 p) (Curry0 q)) :
(PlaneCurveZeroSet p ∩ PlaneCurveZeroSet q).Finite ∧
(PlaneCurveZeroSet p ∩ PlaneCurveZeroSet q).ncard ≤
((d₁ + d₂) ^ 2 + 1) * max 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/Bezout.lean#L225-L368
Human review
Confirmed by the mission captain (proposal self-audit).