<p>In this paper, we seek to investigate this important question: Is it possible that a Drazin invertible operator and its Drazin inverse are not normal, but their sum is normal? Exploring to find the answer to this question made us able to introduce a new class of operators. A Drazin invertible operator <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="74" /> </InlineMediaObject> <EquationSource Format="TEX">\(T \in \mathcal {B}(\mathcal {H})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>T</mi> <mo>∈</mo> <mi mathvariant="script">B</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">H</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is called of class <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="38" /> </InlineMediaObject> <EquationSource Format="TEX">\([\mathfrak {GN}]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mi mathvariant="fraktur">GN</mi> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation> if <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq3.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(T^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>T</mi> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq4.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="58" /> </InlineMediaObject> <EquationSource Format="TEX">\(T + T^D\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>T</mi> <mo>+</mo> <msup> <mi>T</mi> <mi>D</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> commute or equivalently <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq4.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="58" /> </InlineMediaObject> <EquationSource Format="TEX">\(T + T^D\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>T</mi> <mo>+</mo> <msup> <mi>T</mi> <mi>D</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> is normal. This class contains the class of normal operators. First, we give a list of conditions on an operator <i>T</i>,&#xa0; each of which is equivalent to <i>T</i> being of class <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="38" /> </InlineMediaObject> <EquationSource Format="TEX">\([\mathfrak {GN}]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mi mathvariant="fraktur">GN</mi> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation>. We also present some basic properties of these operators. Moreover, we obtain the matrix representation of these operators. We generalize a very famous result on normal operators, due to Kaplansky. Furthermore, we investigate a necessary and sufficient condition for <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq7.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="104" /> </InlineMediaObject> <EquationSource Format="TEX">\(S, T \in \mathcal {M}_{n}( \mathbb {C}) \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>S</mi> <mo>,</mo> <mi>T</mi> <mo>∈</mo> <msub> <mi mathvariant="script">M</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">C</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="113" /> </InlineMediaObject> <EquationSource Format="TEX">\(ST, TS \in [\mathfrak {GN}]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>S</mi> <mi>T</mi> <mo>,</mo> <mi>T</mi> <mi>S</mi> <mo>∈</mo> <mo stretchy="false">[</mo> <mi mathvariant="fraktur">GN</mi> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation>. Finally, we generalize Fuglede-Putnam commutativity theorem for class <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11785_2025_1835_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="38" /> </InlineMediaObject> <EquationSource Format="TEX">\([\mathfrak {GN}]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mi mathvariant="fraktur">GN</mi> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation> of matrices.</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

On the Normality of the Sum of an Operator with its Drazin Inverse

  • Mansour Dana,
  • Ramesh Yousefi,
  • Fateme Kusari

摘要

In this paper, we seek to investigate this important question: Is it possible that a Drazin invertible operator and its Drazin inverse are not normal, but their sum is normal? Exploring to find the answer to this question made us able to introduce a new class of operators. A Drazin invertible operator \(T \in \mathcal {B}(\mathcal {H})\) T B ( H ) is called of class \([\mathfrak {GN}]\) [ GN ] if \(T^*\) T and \(T + T^D\) T + T D commute or equivalently \(T + T^D\) T + T D is normal. This class contains the class of normal operators. First, we give a list of conditions on an operator T,  each of which is equivalent to T being of class \([\mathfrak {GN}]\) [ GN ] . We also present some basic properties of these operators. Moreover, we obtain the matrix representation of these operators. We generalize a very famous result on normal operators, due to Kaplansky. Furthermore, we investigate a necessary and sufficient condition for \(S, T \in \mathcal {M}_{n}( \mathbb {C}) \) S , T M n ( C ) such that \(ST, TS \in [\mathfrak {GN}]\) S T , T S [ GN ] . Finally, we generalize Fuglede-Putnam commutativity theorem for class \([\mathfrak {GN}]\) [ GN ] of matrices.