Intersection bound for a pair of nonassociated irreducible curves
ProvedPachDeZeeuw.Algebraic.irreducible_pair_intersection_boundLet and be irreducible bivariate real polynomials (, ) of total degrees at most and , not associated to each other (). Then their real zero sets meet in a finite set, with an explicit bound:
The proof is a three-way case split on whether the curried polynomials and have degree zero in the eliminated variable. Both zero: the intersection is empty (zeroCurry_zeroCurry_pair_intersection_bound). Exactly one zero: zeroCurry_nonvertical_pair_intersection_bound, applied with the roles of and chosen so that the degree-zero one comes first, gives . Both positive: the curries are primitive and relatively prime, and primitive_nonvertical_pair_intersection_bound gives . Each of these is at most . The associated case, a shared irreducible factor, is excluded here by hypothesis; in factorized_bezout_bound that case is handled separately through finite_real_zero_set_of_irreducible_factor_bound and the singularity count, since the hypothesis there only forbids a shared factor with infinite real zero set.
import Mathlib import Definitions.Def_PdzBezout import Definitions.Def_PdzPrelim open EuclideanGeometry open scoped Topology open PachDeZeeuw.Algebraic open PachDeZeeuw.Algebraic.PlaneCurve
theorem PachDeZeeuw.Algebraic.irreducible_pair_intersection_bound {d₁ : ℕ} {d₂ : ℕ} (h k : MvPolynomial (Fin 2) ℝ)
(hh : Irreducible h) (hk : Irreducible k)
(hdeg : h.totalDegree ≤ d₁)
(kdeg : k.totalDegree ≤ d₂)
(hnot : ¬ Associated h k) :
(PlaneCurveZeroSet h ∩ PlaneCurveZeroSet k).Finite ∧
(PlaneCurveZeroSet h ∩ PlaneCurveZeroSet k).ncard ≤
(d₁ + d₂ + 1) ^ 4 := by sorry
Confirmed by the mission captain (proposal self-audit).