<p>Let <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq1.gif" Format="GIF" Height="15" Rendition="HTML" Resolution="72" Type="Linedraw" Width="43" /> </InlineMediaObject> <EquationSource Format="TEX">\(n\ge 5\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>5</mn> </mrow> </math></EquationSource> </InlineEquation> and let <i>K</i> and <i>L</i> be two convex bodies in <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq2.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="20" /> </InlineMediaObject> <EquationSource Format="TEX">\({{\mathbb {R}}^n}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> such that their orthogonal projections <i>K</i>|<i>G</i> and <i>L</i>|<i>G</i> onto any <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="53" /> </InlineMediaObject> <EquationSource Format="TEX">\((n-1)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo>-</mo> <mn>1</mn> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>-dimensional subspace <i>G</i> are rotations of each other, i.e., there exists a rotation <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq4.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="141" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi _G\in SO(n-1, G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>φ</mi> <mi>G</mi> </msub> <mo>∈</mo> <mi>S</mi> <mi>O</mi> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo>-</mo> <mn>1</mn> <mo>,</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq5.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="124" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi _G(K|G)=L|G\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>φ</mi> <mi>G</mi> </msub> <mrow> <mrow> <mo stretchy="false">(</mo> <mi>K</mi> <mo stretchy="false">|</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>L</mi> <mo stretchy="false">|</mo> <mi>G</mi> </mrow> </mrow> </math></EquationSource> </InlineEquation>. Assume also that the 2-dimensional projections of <i>K</i> and <i>L</i> are pairwise different and they do not have <i>SO</i>(2)-rotational symmetries. Then <i>K</i> and <i>L</i> are congruent. More precisely, we show that <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq6.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="54" /> </InlineMediaObject> <EquationSource Format="TEX">\(K=L\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>=</mo> <mi>L</mi> </mrow> </math></EquationSource> </InlineEquation>, or <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq7.gif" Format="GIF" Height="15" Rendition="HTML" Resolution="72" Type="Linedraw" Width="67" /> </InlineMediaObject> <EquationSource Format="TEX">\(K=-L\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>=</mo> <mo>-</mo> <mi>L</mi> </mrow> </math></EquationSource> </InlineEquation>, or there exists an orthogonal transformation <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="70" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Phi \in O(n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Φ</mi> <mo>∈</mo> <mi>O</mi> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq9.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="77" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Phi (K)=L\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Φ</mi> <mo stretchy="false">(</mo> <mi>K</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>L</mi> </mrow> </math></EquationSource> </InlineEquation>. We also prove an analogous result for sections of star bodies. Both results are the consequences of a more general statement about continuous functions on the unit sphere in <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="605_2025_2073_Article_IEq10.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="20" /> </InlineMediaObject> <EquationSource Format="TEX">\({{\mathbb {R}}^n}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>.</p>

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On a continuous n-dimensional Rubik’s cube without 2-dimensional symmetries

  • Reema A. Sbeih

摘要

Let \(n\ge 5\) n 5 and let K and L be two convex bodies in \({{\mathbb {R}}^n}\) R n such that their orthogonal projections K|G and L|G onto any \((n-1)\) ( n - 1 ) -dimensional subspace G are rotations of each other, i.e., there exists a rotation \(\varphi _G\in SO(n-1, G)\) φ G S O ( n - 1 , G ) such that \(\varphi _G(K|G)=L|G\) φ G ( K | G ) = L | G . Assume also that the 2-dimensional projections of K and L are pairwise different and they do not have SO(2)-rotational symmetries. Then K and L are congruent. More precisely, we show that \(K=L\) K = L , or \(K=-L\) K = - L , or there exists an orthogonal transformation \(\Phi \in O(n)\) Φ O ( n ) such that \(\Phi (K)=L\) Φ ( K ) = L . We also prove an analogous result for sections of star bodies. Both results are the consequences of a more general statement about continuous functions on the unit sphere in \({{\mathbb {R}}^n}\) R n .