<p>We prove that if <i>N</i> points lie in convex position in the plane then they determine <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\Omega (N^{5/4})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">(</mo> <msup> <mi>N</mi> <mrow> <mn>5</mn> <mo stretchy="false">/</mo> <mn>4</mn> </mrow> </msup> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> distinct angles, provided that the points do not lie on a common circle. This is derived from a more general claim that if <i>N</i> points in the convex position in the real plane determine <i>KN</i> distinct angles, then <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(K=\Omega (N^{1/4})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>=</mo> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">(</mo> <msup> <mi>N</mi> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mn>4</mn> </mrow> </msup> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> or <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\Omega (N/K)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">(</mo> <mi>N</mi> <mo stretchy="false">/</mo> <mi>K</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> points are co-circular. The proof makes use of the implicit order one can give to points in convex position and relies on a slightly more general order assumption. The assumption enables one to reduce the issue to counting incidences between points and a multiset of cubic curves, with special attention being paid to the case when the curves are reducible.</p>

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On Distinct Angles in the Plane

  • Sergei V. Konyagin,
  • Jonathan Passant,
  • Misha Rudnev

摘要

We prove that if N points lie in convex position in the plane then they determine \(\Omega (N^{5/4})\) Ω ( N 5 / 4 ) distinct angles, provided that the points do not lie on a common circle. This is derived from a more general claim that if N points in the convex position in the real plane determine KN distinct angles, then \(K=\Omega (N^{1/4})\) K = Ω ( N 1 / 4 ) or \(\Omega (N/K)\) Ω ( N / K ) points are co-circular. The proof makes use of the implicit order one can give to points in convex position and relies on a slightly more general order assumption. The assumption enables one to reduce the issue to counting incidences between points and a multiset of cubic curves, with special attention being paid to the case when the curves are reducible.