<p>Let <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(K\subset \mathbb {R}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>⊂</mo> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> be a convex body, <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(n\ge 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>. We say that <i>K</i> satisfies the <i>Barker-Larman condition</i> if there exists a ball <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(B\subset \text {int} K\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>B</mi> <mo>⊂</mo> <mtext>int</mtext> <mi>K</mi> </mrow> </math></EquationSource> </InlineEquation> such that for every support hyperplane <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\Pi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Π</mi> </math></EquationSource> </InlineEquation> of <i>B</i>, the section <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\Pi \cap K\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Π</mi> <mo>∩</mo> <mi>K</mi> </mrow> </math></EquationSource> </InlineEquation> is a centrally symmetric set. In [<CitationRef CitationID="CR4">4</CitationRef>], it was conjectured that the Barker-Larman condition characterizes the ellipsoid. In this work, we prove a special case of such a conjecture; in particular, we assume that the convex body <i>K</i> is centrally symmetric. Our main result is the following: Let <i>K</i> be a centrally symmetric and strictly convex body, with center at <i>O</i>, and let <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(B\subset \operatorname {int}K\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>B</mi> <mo>⊂</mo> <mo>int</mo> <mi>K</mi> </mrow> </math></EquationSource> </InlineEquation> be a ball not containing <i>O</i>: If <i>K</i> satisfies the Barker-Larman condition with respect to <i>B</i> and <i>B</i> is <i>suitable</i> for <i>K</i> (intuitively, <i>B</i> is suitable for <i>K</i> if <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\operatorname {bd}B\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>bd</mo> <mi>B</mi> </mrow> </math></EquationSource> </InlineEquation> <i>is not very close to</i> <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\operatorname {bd}K\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>bd</mo> <mi>K</mi> </mrow> </math></EquationSource> </InlineEquation>, see the definition in the Introduction), then <i>K</i> is an ellipsoid.</p>

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Convex bodies with centrally symmetric sections

  • E. Morales-Amaya

摘要

Let \(K\subset \mathbb {R}^n\) K R n be a convex body, \(n\ge 3\) n 3 . We say that K satisfies the Barker-Larman condition if there exists a ball \(B\subset \text {int} K\) B int K such that for every support hyperplane \(\Pi \) Π of B, the section \(\Pi \cap K\) Π K is a centrally symmetric set. In [4], it was conjectured that the Barker-Larman condition characterizes the ellipsoid. In this work, we prove a special case of such a conjecture; in particular, we assume that the convex body K is centrally symmetric. Our main result is the following: Let K be a centrally symmetric and strictly convex body, with center at O, and let \(B\subset \operatorname {int}K\) B int K be a ball not containing O: If K satisfies the Barker-Larman condition with respect to B and B is suitable for K (intuitively, B is suitable for K if \(\operatorname {bd}B\) bd B is not very close to \(\operatorname {bd}K\) bd K , see the definition in the Introduction), then K is an ellipsoid.