<p>Let <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(S^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>S</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> be the <i>n</i>-sphere with the geodesic metric and of diameter <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\pi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>π</mi> </math></EquationSource> </InlineEquation>. The intrinsic Čech complex <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\check{\textrm{C}}(S^n;r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mtext>C</mtext> <mo stretchy="false">ˇ</mo> </mover> <mrow> <mo stretchy="false">(</mo> <msup> <mi>S</mi> <mi>n</mi> </msup> <mo>;</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is the nerve of all open balls of radius <i>r</i> in <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(S^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>S</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>. In this paper, we show how to control the homotopy connectivity of <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\check{\textrm{C}}(S^n;r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mtext>C</mtext> <mo stretchy="false">ˇ</mo> </mover> <mrow> <mo stretchy="false">(</mo> <msup> <mi>S</mi> <mi>n</mi> </msup> <mo>;</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> in terms of coverings of spheres. Our upper bound on the connectivity, which is sharp in the case <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(n=1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>=</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>, comes from the chromatic numbers of Borsuk graphs of spheres. Our lower bound is obtained using the conicity (in the sense of Barmak) of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\check{\textrm{C}}(X;r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mtext>C</mtext> <mo stretchy="false">ˇ</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo>;</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> for sufficiently dense, finite subsets <i>X</i> of <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(S^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>S</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>. Our bounds imply the new result that for <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(n\ge 1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>, the homotopy type of <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\check{\textrm{C}}(S^n;r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mtext>C</mtext> <mo stretchy="false">ˇ</mo> </mover> <mrow> <mo stretchy="false">(</mo> <msup> <mi>S</mi> <mi>n</mi> </msup> <mo>;</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> changes infinitely many times as <i>r</i> varies over <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\((0,\pi )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mn>0</mn> <mo>,</mo> <mi>π</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>; we conjecture only countably many times. Additionally, we lower bound the homological dimension of Čech complexes of finite subsets <i>X</i> of <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(S^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>S</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> in terms of packings of <i>X</i>.</p>

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Homotopy Connectivity of Čech Complexes of Spheres

  • Henry Adams,
  • Ekansh Jauhari,
  • Sucharita Mallick

摘要

Let \(S^n\) S n be the n-sphere with the geodesic metric and of diameter \(\pi \) π . The intrinsic Čech complex \(\check{\textrm{C}}(S^n;r)\) C ˇ ( S n ; r ) is the nerve of all open balls of radius r in \(S^n\) S n . In this paper, we show how to control the homotopy connectivity of \(\check{\textrm{C}}(S^n;r)\) C ˇ ( S n ; r ) in terms of coverings of spheres. Our upper bound on the connectivity, which is sharp in the case \(n=1\) n = 1 , comes from the chromatic numbers of Borsuk graphs of spheres. Our lower bound is obtained using the conicity (in the sense of Barmak) of \(\check{\textrm{C}}(X;r)\) C ˇ ( X ; r ) for sufficiently dense, finite subsets X of \(S^n\) S n . Our bounds imply the new result that for \(n\ge 1\) n 1 , the homotopy type of \(\check{\textrm{C}}(S^n;r)\) C ˇ ( S n ; r ) changes infinitely many times as r varies over \((0,\pi )\) ( 0 , π ) ; we conjecture only countably many times. Additionally, we lower bound the homological dimension of Čech complexes of finite subsets X of \(S^n\) S n in terms of packings of X.