<p>In this paper, we prove that if a commuting family of operators <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10476_2025_111_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="85" /> </InlineMediaObject> <EquationSource Format="TEX">\( \tau = (T_\lambda)_{\lambda\in\Lambda}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>τ</mi> <mo>=</mo> <msub> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mi>λ</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mrow> <mi>λ</mi> <mo>∈</mo> <mi mathvariant="normal">Λ</mi> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation> on a Banach space <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10476_2025_111_Article_IEq2.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal{X}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">X</mi> </math></EquationSource> </InlineEquation> is Bochner integrable, then<Equation ID="Equ1"> <MediaObject> <ImageObject Color="BlackWhite" FileRef="10476_2025_111_Article_Equ1.gif" Format="GIF" Height="43" Rendition="HTML" Resolution="72" Type="Linedraw" Width="248" /> </MediaObject> <EquationSource Format="TEX">\(r\left({}^{^{B}}\!\!\!\!\int_{\Lambda}T_{\lambda}d\mu(\lambda) \right) \leq \int_\Lambda r(T_\lambda)\,d\mu(\lambda).\)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mi>r</mi> <mfenced close=")" open="("> <mmultiscripts> <mrow /> <mrow /> <mmultiscripts> <mrow /> <mrow /> <mi>B</mi> </mmultiscripts> </mmultiscripts> <mspace width="-0.166667em" /> <mspace width="-0.166667em" /> <mspace width="-0.166667em" /> <mspace width="-0.166667em" /> <msub> <mo>∫</mo> <mi mathvariant="normal">Λ</mi> </msub> <msub> <mi>T</mi> <mi>λ</mi> </msub> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>λ</mi> <mo stretchy="false">)</mo> </mrow> </mfenced> <mo>≤</mo> <msub> <mo>∫</mo> <mi mathvariant="normal">Λ</mi> </msub> <mi>r</mi> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mi>λ</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mspace width="0.166667em" /> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>λ</mi> <mo stretchy="false">)</mo> </mrow> <mo>.</mo> </mrow> </math></EquationSource> </Equation>This result extends the well-known theorem on the subadditivity of the spectral radius for a finite set of commuting operators. We also provide an example illustrating that Bochner integrability cannot be substituted by a weaker form of integrability.</p>

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Spectral radius subadditivity for integrals of operator-valued functions

  • H. Stanković,
  • M. Krstić

摘要

In this paper, we prove that if a commuting family of operators \( \tau = (T_\lambda)_{\lambda\in\Lambda}\) τ = ( T λ ) λ Λ on a Banach space \(\mathcal{X}\) X is Bochner integrable, then \(r\left({}^{^{B}}\!\!\!\!\int_{\Lambda}T_{\lambda}d\mu(\lambda) \right) \leq \int_\Lambda r(T_\lambda)\,d\mu(\lambda).\) r B Λ T λ d μ ( λ ) Λ r ( T λ ) d μ ( λ ) . This result extends the well-known theorem on the subadditivity of the spectral radius for a finite set of commuting operators. We also provide an example illustrating that Bochner integrability cannot be substituted by a weaker form of integrability.