Dilation pullback: $P_\beta(cX,\bar cY)=k\,P_\alpha$ , $|\alpha|=|\beta|=1$ , forces $|c|=1$ and $\beta=\alpha$

Proved

CurveSymmetry.family_dilation_parameter

by carlok · Oct 7, 2026 · Mathlib 0df444a (Lean v4.33.1)

algebraic-geometrycurve-symmetry-leaneuclidean-geometry

Let $m\ge1$ be an integer and let $\alpha,\beta,c,k\in\mathbb C$ with $|\alpha|=|\beta|=1$ and $c\ne0$ . For $\gamma\in\mathbb C$ let $P_\gamma(X,Y)=X^m(\gamma+XY)+Y^m(\bar\gamma+XY)$ be the polynomial of equation (6). The dilation $z\mapsto cz$ acts on the complex coordinates $X=z$ , $Y=\bar z$ by $X\mapsto cX$ , $Y\mapsto\bar cY$ .

Suppose that the pullback of $P_\beta$ under this dilation is a multiple of $P_\alpha$ :

P_\beta(cX,\bar cY)=k\,P_\alpha(X,Y)\qquad\text{in }\mathbb C[X,Y].

Then $|c|=1$ and $\beta=\alpha$ .

This is the case $z\mapsto cz$ of the coefficient-ratio comparison (8) in the proof of Theorem 2, where the ratio is $\beta/|c|^2$ : such a dilation must be a rotation and cannot change a parameter of modulus one.

Preamble

import Definitions.Def_CurveSymmetry_01_RealLoci
import Definitions.Def_CurveSymmetry_02_FamilyBasics
import Definitions.Def_CurveSymmetry_03_IsometriesAndCharts
import Mathlib.Algebra.MvPolynomial.Nilpotent
import Mathlib.Algebra.MvPolynomial.NoZeroDivisors
import Mathlib.Algebra.MvPolynomial.PDeriv
import Mathlib.Algebra.Polynomial.FieldDivision
import Mathlib.Algebra.Polynomial.Reverse
import Mathlib.Analysis.Complex.Isometry
import Mathlib.Analysis.Complex.OperatorNorm
import Mathlib.Analysis.Complex.Polynomial.Basic
import Mathlib.Analysis.Normed.Affine.MazurUlam
import Mathlib.Data.Complex.Basic
import Mathlib.FieldTheory.Separable
import Mathlib.RingTheory.MvPolynomial.Homogeneous
import Mathlib.RingTheory.MvPolynomial.IrreducibleQuadratic
import Mathlib.RingTheory.Polynomial.Eisenstein.Criterion
import Mathlib.RingTheory.Polynomial.GaussLemma
import Mathlib.RingTheory.RootsOfUnity.Complex
import Mathlib.RingTheory.RootsOfUnity.PrimitiveRoots
import Mathlib.Tactic

open CurveSymmetry
set_option autoImplicit false
open MvPolynomial

Formal statement

theorem CurveSymmetry.family_dilation_parameter {m : ℕ} (hm : 0 < m) {α β c k : ℂ}
    (hα : ‖α‖ = 1) (hβ : ‖β‖ = 1) (hc : c ≠ 0)
    (he : dilate c (familyPolynomial m β) = C k * familyPolynomial m α) :
    ‖c‖ = 1 ∧ β = α := by sorry

Source

C. Perassi, Sharp symmetry bounds for real algebraic curves (note): lemma of the formalization, for proof of Theorem 2, equation (8), p. 4, https://github.com/carlok/curve-symmetry-lean/blob/d99bc17a1c397956c05d7417de50f9beed56580f/sharp_symmetry_bounds.pdf. Lean: https://github.com/carlok/curve-symmetry-lean/blob/d99bc17a1c397956c05d7417de50f9beed56580f/lean/FamilyTransport.lean (C. Perassi)

View graph

Dilation pullback: Pβ(cX,cˉY)=k PαP_\beta(cX,\bar cY)=k\,P_\alphaPβ​(cX,cˉY)=kPα​, ∣α∣=∣β∣=1|\alpha|=|\beta|=1∣α∣=∣β∣=1, forces ∣c∣=1|c|=1∣c∣=1 and β=α\beta=\alphaβ=α

Dilation pullback: $P_\beta(cX,\bar cY)=k\,P_\alpha$ , $|\alpha|=|\beta|=1$ , forces $|c|=1$ and $\beta=\alpha$