<p>A distinguished variety in the polydisc <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> is an affine complex algebraic variety that intersects <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> and exits the domain through the <i>n</i>-torus <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq3.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="20" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb T^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">T</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> without intersecting any other part of the topological boundary of <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>. We find two different characterizations for a distinguished variety in the polydisc <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> in terms of the Taylor joint spectrum of certain linear matrix-pencils and thus generalize the seminal work due to Agler and McCarthy [Acta Math., 2005] on distinguished varieties in <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq6.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^2\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mn>2</mn> </msup> </math></EquationSource> </InlineEquation>. We show that a distinguished variety in <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> is a part of an affine algebraic curve which is a set-theoretic complete intersection. We also show that if <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="85" /> </InlineMediaObject> <EquationSource Format="TEX">\((T_1, \dots , T_n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>T</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is commuting tuple of Hilbert space contractions such that the defect space of <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq9.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="90" /> </InlineMediaObject> <EquationSource Format="TEX">\(T=\prod _{i=1}^n T_i\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>T</mi> <mo>=</mo> <msubsup> <mo>∏</mo> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </msubsup> <msub> <mi>T</mi> <mi>i</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is finite dimensional, then <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="85" /> </InlineMediaObject> <EquationSource Format="TEX">\((T_1, \dots , T_n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>T</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> admits a commuting unitary dilation <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq11.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="88" /> </InlineMediaObject> <EquationSource Format="TEX">\((U_1, \dots , U_n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>U</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>U</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq12.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="93" /> </InlineMediaObject> <EquationSource Format="TEX">\(U=\prod _{i=1}^n U_i\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>U</mi> <mo>=</mo> <msubsup> <mo>∏</mo> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </msubsup> <msub> <mi>U</mi> <mi>i</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> being the minimal unitary dilation of <i>T</i> if and only if some certain matrices associated with <InlineEquation ID="IEq13"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="85" /> </InlineMediaObject> <EquationSource Format="TEX">\((T_1, \dots , T_n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>T</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> define a distinguished variety in <InlineEquation ID="IEq14"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3793_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb D^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi mathvariant="double-struck">D</mi> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>.</p>

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Distinguished varieties in the polydisc and dilation of commuting contractions

  • Sourav Pal

摘要

A distinguished variety in the polydisc \(\mathbb D^n\) D n is an affine complex algebraic variety that intersects \(\mathbb D^n\) D n and exits the domain through the n-torus \(\mathbb T^n\) T n without intersecting any other part of the topological boundary of \(\mathbb D^n\) D n . We find two different characterizations for a distinguished variety in the polydisc \(\mathbb D^n\) D n in terms of the Taylor joint spectrum of certain linear matrix-pencils and thus generalize the seminal work due to Agler and McCarthy [Acta Math., 2005] on distinguished varieties in \(\mathbb D^2\) D 2 . We show that a distinguished variety in \(\mathbb D^n\) D n is a part of an affine algebraic curve which is a set-theoretic complete intersection. We also show that if \((T_1, \dots , T_n)\) ( T 1 , , T n ) is commuting tuple of Hilbert space contractions such that the defect space of \(T=\prod _{i=1}^n T_i\) T = i = 1 n T i is finite dimensional, then \((T_1, \dots , T_n)\) ( T 1 , , T n ) admits a commuting unitary dilation \((U_1, \dots , U_n)\) ( U 1 , , U n ) with \(U=\prod _{i=1}^n U_i\) U = i = 1 n U i being the minimal unitary dilation of T if and only if some certain matrices associated with \((T_1, \dots , T_n)\) ( T 1 , , T n ) define a distinguished variety in \(\mathbb D^n\) D n .