<p>Let <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mathscr {L}(\mathscr {H})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">H</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> be the algebra of all bounded linear operators acting on a complex Hilbert space of dimension greater than 2,&#xa0; <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathscr {A}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">A</mi> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathscr {B}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">B</mi> </math></EquationSource> </InlineEquation> be subsets of <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mathscr {L}(\mathscr {H})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">H</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> which contain all operators of rank at most one. For <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(0&lt; \varepsilon &lt; 1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>0</mn> <mo>&lt;</mo> <mi>ε</mi> <mo>&lt;</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(A\in \mathscr {L}(\mathscr {H}),\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo>∈</mo> <mi mathvariant="script">L</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">H</mi> <mo stretchy="false">)</mo> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> we denote by <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\sigma _{\varepsilon }(A)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>σ</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> (respectively, by <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(r_{\varepsilon }(A)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>r</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>) the <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\varepsilon \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>ε</mi> </math></EquationSource> </InlineEquation>-condition spectrum (respectively, the <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\varepsilon \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>ε</mi> </math></EquationSource> </InlineEquation>-condition spectral radius) of <i>A</i>. We prove that surjective maps <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\phi _1, \phi _2:\mathscr {A} \longrightarrow \mathscr {B}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>ϕ</mi> <mn>1</mn> </msub> <mo>,</mo> <msub> <mi>ϕ</mi> <mn>2</mn> </msub> <mo>:</mo> <mi mathvariant="script">A</mi> <mo stretchy="false">⟶</mo> <mi mathvariant="script">B</mi> </mrow> </math></EquationSource> </InlineEquation> satisfy <Equation ID="Equ46"> <EquationSource Format="TEX">\( \sigma _{\varepsilon }(\phi _1(A)\phi _2(B)) = \sigma _{\varepsilon }(AB) \hspace{0.2cm} \left( A,B\in \mathscr {A} \right) , \)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <msub> <mi>σ</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msub> <mi>ϕ</mi> <mn>1</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi>ϕ</mi> <mn>2</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>B</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msub> <mi>σ</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mi>B</mi> <mo stretchy="false">)</mo> </mrow> <mspace width="5.69046pt" /> <mfenced close=")" open="("> <mi>A</mi> <mo>,</mo> <mi>B</mi> <mo>∈</mo> <mi mathvariant="script">A</mi> </mfenced> <mo>,</mo> </mrow> </math></EquationSource> </Equation>if and only if there exist a bounded invertible linear operator <i>U</i> on <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathscr {H}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">H</mi> </math></EquationSource> </InlineEquation> and a unitary operator <i>V</i> on <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\mathscr {H}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">H</mi> </math></EquationSource> </InlineEquation> such that, for all <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(A\in \mathscr {A}, \phi _1(A) = s VAU^{-1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo>∈</mo> <mi mathvariant="script">A</mi> <mo>,</mo> <msub> <mi>ϕ</mi> <mn>1</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>s</mi> <mi>V</mi> <mi>A</mi> <msup> <mi>U</mi> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(\phi _2(A) = r UAV^*\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>ϕ</mi> <mn>2</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>r</mi> <mi>U</mi> <mi>A</mi> <msup> <mi>V</mi> <mo>∗</mo> </msup> </mrow> </math></EquationSource> </InlineEquation> for some <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\(r, s\in \mathbb {C}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>r</mi> <mo>,</mo> <mi>s</mi> <mo>∈</mo> <mi mathvariant="double-struck">C</mi> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq17"> <EquationSource Format="TEX">\(rs=1.\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>r</mi> <mi>s</mi> <mo>=</mo> <mn>1</mn> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation> We also characterize all surjective maps <InlineEquation ID="IEq18"> <EquationSource Format="TEX">\(\phi _1, \phi _2:\mathscr {A} \longrightarrow \mathscr {B}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>ϕ</mi> <mn>1</mn> </msub> <mo>,</mo> <msub> <mi>ϕ</mi> <mn>2</mn> </msub> <mo>:</mo> <mi mathvariant="script">A</mi> <mo stretchy="false">⟶</mo> <mi mathvariant="script">B</mi> </mrow> </math></EquationSource> </InlineEquation> which satisfy <Equation ID="Equ47"> <EquationSource Format="TEX">\( r_{\varepsilon }(\phi _1(A)\phi _2(B)) = r_{\varepsilon }(AB) \hspace{0.2cm} \left( A,B\in \mathscr {A} \right) . \)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <msub> <mi>r</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msub> <mi>ϕ</mi> <mn>1</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi>ϕ</mi> <mn>2</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>B</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msub> <mi>r</mi> <mi>ε</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mi>B</mi> <mo stretchy="false">)</mo> </mrow> <mspace width="5.69046pt" /> <mfenced close=")" open="("> <mi>A</mi> <mo>,</mo> <mi>B</mi> <mo>∈</mo> <mi mathvariant="script">A</mi> </mfenced> <mo>.</mo> </mrow> </math></EquationSource> </Equation></p>

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On the condition spectrum preservers

  • Hamid Nkhaylia

摘要

Let \(\mathscr {L}(\mathscr {H})\) L ( H ) be the algebra of all bounded linear operators acting on a complex Hilbert space of dimension greater than 2,  \(\mathscr {A}\) A and \(\mathscr {B}\) B be subsets of \(\mathscr {L}(\mathscr {H})\) L ( H ) which contain all operators of rank at most one. For \(0< \varepsilon < 1\) 0 < ε < 1 and \(A\in \mathscr {L}(\mathscr {H}),\) A L ( H ) , we denote by \(\sigma _{\varepsilon }(A)\) σ ε ( A ) (respectively, by \(r_{\varepsilon }(A)\) r ε ( A ) ) the \(\varepsilon \) ε -condition spectrum (respectively, the \(\varepsilon \) ε -condition spectral radius) of A. We prove that surjective maps \(\phi _1, \phi _2:\mathscr {A} \longrightarrow \mathscr {B}\) ϕ 1 , ϕ 2 : A B satisfy \( \sigma _{\varepsilon }(\phi _1(A)\phi _2(B)) = \sigma _{\varepsilon }(AB) \hspace{0.2cm} \left( A,B\in \mathscr {A} \right) , \) σ ε ( ϕ 1 ( A ) ϕ 2 ( B ) ) = σ ε ( A B ) A , B A , if and only if there exist a bounded invertible linear operator U on \(\mathscr {H}\) H and a unitary operator V on \(\mathscr {H}\) H such that, for all \(A\in \mathscr {A}, \phi _1(A) = s VAU^{-1}\) A A , ϕ 1 ( A ) = s V A U - 1 and \(\phi _2(A) = r UAV^*\) ϕ 2 ( A ) = r U A V for some \(r, s\in \mathbb {C}\) r , s C with \(rs=1.\) r s = 1 . We also characterize all surjective maps \(\phi _1, \phi _2:\mathscr {A} \longrightarrow \mathscr {B}\) ϕ 1 , ϕ 2 : A B which satisfy \( r_{\varepsilon }(\phi _1(A)\phi _2(B)) = r_{\varepsilon }(AB) \hspace{0.2cm} \left( A,B\in \mathscr {A} \right) . \) r ε ( ϕ 1 ( A ) ϕ 2 ( B ) ) = r ε ( A B ) A , B A .