<p>In this work, we study the geometry of the unit ball of the space of operators <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({\mathcal {L}}(X,Y^*)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, by considering the projective tensor product <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(X\hat{\otimes }_{\pi } Y\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>X</mi> <msub> <mover accent="true"> <mo>⊗</mo> <mo stretchy="false">^</mo> </mover> <mi>π</mi> </msub> <mi>Y</mi> </mrow> </math></EquationSource> </InlineEquation> as a predual. We prove that if an elementary tensor (rank one operator) of the form <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(x_0^*\otimes y_0^* \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>x</mi> <mn>0</mn> <mo>∗</mo> </msubsup> <mo>⊗</mo> <msubsup> <mi>y</mi> <mn>0</mn> <mo>∗</mo> </msubsup> </mrow> </math></EquationSource> </InlineEquation> in the unit sphere <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\( S_{{\mathcal {L}}(X,Y^*)}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>S</mi> <mrow> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> </msub> </math></EquationSource> </InlineEquation> is a <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-strongly extreme point of the unit ball, then <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(x_0^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mi>x</mi> <mn>0</mn> <mo>∗</mo> </msubsup> </math></EquationSource> </InlineEquation> is <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-strongly extreme point of unit ball of <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(X^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>X</mi> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(y_0^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mi>y</mi> <mn>0</mn> <mo>∗</mo> </msubsup> </math></EquationSource> </InlineEquation> is <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-strongly extreme point of the unit ball of <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(Y^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>Y</mi> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>. We show that a similar conclusion holds if the rank one operator is a Namioka point (equivalently, a point of <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-weak continuity for the identity mapping) on the unit sphere of <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\({\mathcal {L}}(X,Y^*)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. We also study extremal phenomena in the unit ball of <InlineEquation ID="IEq17"> <EquationSource Format="TEX">\({\mathcal {L}}(X,Y^*)^*\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <msup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> <mo>∗</mo> </msup> </mrow> </math></EquationSource> </InlineEquation>. We partly solve the open problem: when does an elementary tensor, whose components are Namioka points, become a Namioka point again? We show that if a point <InlineEquation ID="IEq18"> <EquationSource Format="TEX">\(z\in S_{{\mathcal {L}}(X,Y^*)^*}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>z</mi> <mo>∈</mo> <msub> <mi>S</mi> <mrow> <mi mathvariant="script">L</mi> <msup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> <mo>∗</mo> </msup> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation> is a <InlineEquation ID="IEq19"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-strongly extreme point of the unit ball, then <InlineEquation ID="IEq20"> <EquationSource Format="TEX">\(z=x\otimes y\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>z</mi> <mo>=</mo> <mi>x</mi> <mo>⊗</mo> <mi>y</mi> </mrow> </math></EquationSource> </InlineEquation> for some <InlineEquation ID="IEq21"> <EquationSource Format="TEX">\(\hbox {weak}^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mtext>weak</mtext> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation>-strongly extreme points <InlineEquation ID="IEq22"> <EquationSource Format="TEX">\(x\in S_X\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>x</mi> <mo>∈</mo> <msub> <mi>S</mi> <mi>X</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq23"> <EquationSource Format="TEX">\(y\in S_Y\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>y</mi> <mo>∈</mo> <msub> <mi>S</mi> <mi>Y</mi> </msub> </mrow> </math></EquationSource> </InlineEquation>, provided the space of compact operators, <InlineEquation ID="IEq24"> <EquationSource Format="TEX">\(\mathcal {K}(X,Y^*)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">K</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo>,</mo> <msup> <mi>Y</mi> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is separating for <InlineEquation ID="IEq25"> <EquationSource Format="TEX">\(X\hat{\otimes }_{\pi } Y\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>X</mi> <msub> <mover accent="true"> <mo>⊗</mo> <mo stretchy="false">^</mo> </mover> <mi>π</mi> </msub> <mi>Y</mi> </mrow> </math></EquationSource> </InlineEquation>.</p>

错误:搜索内容不能为空,请输入英文关键词
错误:关键词超出字数限制,请精简
高级检索

Geometry of the unit ball of \({\mathcal {L}}(X,Y^*)\)

  • T. S. S. R. K. Rao,
  • Susmita Seal

摘要

In this work, we study the geometry of the unit ball of the space of operators \({\mathcal {L}}(X,Y^*)\) L ( X , Y ) , by considering the projective tensor product \(X\hat{\otimes }_{\pi } Y\) X ^ π Y as a predual. We prove that if an elementary tensor (rank one operator) of the form \(x_0^*\otimes y_0^* \) x 0 y 0 in the unit sphere \( S_{{\mathcal {L}}(X,Y^*)}\) S L ( X , Y ) is a \(\hbox {weak}^*\) weak -strongly extreme point of the unit ball, then \(x_0^*\) x 0 is \(\hbox {weak}^*\) weak -strongly extreme point of unit ball of \(X^*\) X and \(y_0^*\) y 0 is \(\hbox {weak}^*\) weak -strongly extreme point of the unit ball of \(Y^*\) Y . We show that a similar conclusion holds if the rank one operator is a Namioka point (equivalently, a point of \(\hbox {weak}^*\) weak -weak continuity for the identity mapping) on the unit sphere of \({\mathcal {L}}(X,Y^*)\) L ( X , Y ) . We also study extremal phenomena in the unit ball of \({\mathcal {L}}(X,Y^*)^*\) L ( X , Y ) . We partly solve the open problem: when does an elementary tensor, whose components are Namioka points, become a Namioka point again? We show that if a point \(z\in S_{{\mathcal {L}}(X,Y^*)^*}\) z S L ( X , Y ) is a \(\hbox {weak}^*\) weak -strongly extreme point of the unit ball, then \(z=x\otimes y\) z = x y for some \(\hbox {weak}^*\) weak -strongly extreme points \(x\in S_X\) x S X and \(y\in S_Y\) y S Y , provided the space of compact operators, \(\mathcal {K}(X,Y^*)\) K ( X , Y ) is separating for \(X\hat{\otimes }_{\pi } Y\) X ^ π Y .