<p>We use the correlation matrix of the generating distribution to determine the mixing time for random walks on the torus <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\((\mathbb {Z}/q\mathbb {Z})^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">Z</mi> <mo stretchy="false">/</mo> <mi>q</mi> <mi mathvariant="double-struck">Z</mi> <mo stretchy="false">)</mo> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>. We present our method in the context of the Diaconis–Gangolli random walk on both the <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(1 \times n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo>×</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(m \times n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>m</mi> <mo>×</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation> contingency tables over <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mathbb {Z}/q\mathbb {Z}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">Z</mi> <mo stretchy="false">/</mo> <mi>q</mi> <mi mathvariant="double-struck">Z</mi> </mrow> </math></EquationSource> </InlineEquation>. In the <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(1 \times n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo>×</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation> case, we prove that the random walk exhibits cutoff at time <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\dfrac{n q^2 \log (n)}{8 \pi ^2}\)</EquationSource> <EquationSource Format="MATHML"><math> <mstyle displaystyle="true" scriptlevel="0"> <mfrac> <mrow> <mi>n</mi> <msup> <mi>q</mi> <mn>2</mn> </msup> <mo>log</mo> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mn>8</mn> <msup> <mi>π</mi> <mn>2</mn> </msup> </mrow> </mfrac> </mstyle> </math></EquationSource> </InlineEquation> when <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(q \gg n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>q</mi> <mo>≫</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation>; in the <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(m \times n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>m</mi> <mo>×</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation> case, where <i>m</i> and <i>n</i> are of the same order, we establish cutoff for the random walk at time <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\dfrac{mn q^2 \log (mn)}{16 \pi ^2}\)</EquationSource> <EquationSource Format="MATHML"><math> <mstyle displaystyle="true" scriptlevel="0"> <mfrac> <mrow> <mi>m</mi> <mi>n</mi> <msup> <mi>q</mi> <mn>2</mn> </msup> <mo>log</mo> <mrow> <mo stretchy="false">(</mo> <mi>m</mi> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mn>16</mn> <msup> <mi>π</mi> <mn>2</mn> </msup> </mrow> </mfrac> </mstyle> </math></EquationSource> </InlineEquation> when <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(q \gg n^2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>q</mi> <mo>≫</mo> <msup> <mi>n</mi> <mn>2</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. Our method reveals that a general class of random walks on the torus <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\((\mathbb {Z}/q\mathbb {Z})^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">Z</mi> <mo stretchy="false">/</mo> <mi>q</mi> <mi mathvariant="double-struck">Z</mi> <mo stretchy="false">)</mo> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> has cutoff. If each coordinate of the lifted random walk onto <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathbb {Z}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">Z</mi> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> has variance <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\sigma ^2/n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mi>σ</mi> <mn>2</mn> </msup> <mo stretchy="false">/</mo> <mi>n</mi> </mrow> </math></EquationSource> </InlineEquation> in each jump, and the between-coordinate correlations are sufficiently low, then cutoff occurs at time <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(\dfrac{nq^2 \log (n)}{4\pi ^2 \sigma ^2}\)</EquationSource> <EquationSource Format="MATHML"><math> <mstyle displaystyle="true" scriptlevel="0"> <mfrac> <mrow> <mi>n</mi> <msup> <mi>q</mi> <mn>2</mn> </msup> <mo>log</mo> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </mrow> <mrow> <mn>4</mn> <msup> <mi>π</mi> <mn>2</mn> </msup> <msup> <mi>σ</mi> <mn>2</mn> </msup> </mrow> </mfrac> </mstyle> </math></EquationSource> </InlineEquation>.</p>

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Cutoff for Contingency Table and Torus Random Walks with Low Incremental Correlations

  • Zihao Fang,
  • Andrew Heeszel

摘要

We use the correlation matrix of the generating distribution to determine the mixing time for random walks on the torus \((\mathbb {Z}/q\mathbb {Z})^n\) ( Z / q Z ) n . We present our method in the context of the Diaconis–Gangolli random walk on both the \(1 \times n\) 1 × n and \(m \times n\) m × n contingency tables over \(\mathbb {Z}/q\mathbb {Z}\) Z / q Z . In the \(1 \times n\) 1 × n case, we prove that the random walk exhibits cutoff at time \(\dfrac{n q^2 \log (n)}{8 \pi ^2}\) n q 2 log ( n ) 8 π 2 when \(q \gg n\) q n ; in the \(m \times n\) m × n case, where m and n are of the same order, we establish cutoff for the random walk at time \(\dfrac{mn q^2 \log (mn)}{16 \pi ^2}\) m n q 2 log ( m n ) 16 π 2 when \(q \gg n^2\) q n 2 . Our method reveals that a general class of random walks on the torus \((\mathbb {Z}/q\mathbb {Z})^n\) ( Z / q Z ) n has cutoff. If each coordinate of the lifted random walk onto \(\mathbb {Z}^n\) Z n has variance \(\sigma ^2/n\) σ 2 / n in each jump, and the between-coordinate correlations are sufficiently low, then cutoff occurs at time \(\dfrac{nq^2 \log (n)}{4\pi ^2 \sigma ^2}\) n q 2 log ( n ) 4 π 2 σ 2 .