Closed operator, closed graph












0












$begingroup$


From a course based on Kreyszig's Introduction to Functional analysis.



Let $X neq {0}$ denote a complex normed vector space, and
assume that the operator $T : D(T) ⊂ X → X$ is closed. Let $λ ∈ C$. Show
that the operator $T − λI$ is closed. I have tried applying theorem $4.13-3$, but am unsure of how to choose a sequence and showing convergence.










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$endgroup$

















    0












    $begingroup$


    From a course based on Kreyszig's Introduction to Functional analysis.



    Let $X neq {0}$ denote a complex normed vector space, and
    assume that the operator $T : D(T) ⊂ X → X$ is closed. Let $λ ∈ C$. Show
    that the operator $T − λI$ is closed. I have tried applying theorem $4.13-3$, but am unsure of how to choose a sequence and showing convergence.










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      From a course based on Kreyszig's Introduction to Functional analysis.



      Let $X neq {0}$ denote a complex normed vector space, and
      assume that the operator $T : D(T) ⊂ X → X$ is closed. Let $λ ∈ C$. Show
      that the operator $T − λI$ is closed. I have tried applying theorem $4.13-3$, but am unsure of how to choose a sequence and showing convergence.










      share|cite|improve this question











      $endgroup$




      From a course based on Kreyszig's Introduction to Functional analysis.



      Let $X neq {0}$ denote a complex normed vector space, and
      assume that the operator $T : D(T) ⊂ X → X$ is closed. Let $λ ∈ C$. Show
      that the operator $T − λI$ is closed. I have tried applying theorem $4.13-3$, but am unsure of how to choose a sequence and showing convergence.







      real-analysis functional-analysis operator-theory closed-graph






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 4 '18 at 11:35







      thaumoctopus

















      asked Dec 4 '18 at 10:57









      thaumoctopusthaumoctopus

      9519




      9519






















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          $begingroup$

          Note that for any $lambdainmathbb{C}$, $D(T)=D(T-lambda I)$ and both of $T$ and $I$ are closed. Also, for any two closed operators $T, F$ their linear sum is also closed.



          For proof, you simply consider $T:D(T)to X$ and $F:D(F)to X$ be two closed operators then their corresponding graphs are closed i.e $G_T, G_F$ are closed. We now show $G_{T+F}$ is closed which will imply $T+F:D(T)cap D(F)to X$ defined by $(T+F)x=T(x)+F(x)$is closed.



          For this let $(x,y)in overline{G_{F+T}}$ then $exists$ a sequence ${(x_n,y_n)}_{ninmathbb{N}}$ in $G_{F+T}$ converging to $(x,y)$ with $y_n=(T+F)(x_n),forall ninmathbb{N}implies y_n=T(x_n)+F(x_n),forall ninmathbb{N}$.



          Since $G_T, G_F$ are both closed so $T(x_n)to T(x)$, $F(x_n)to F(x)$ as $x_nto x$ as $nto infty$. Hence, $y_n=T(x_n)+F(x_n)to T(x)+F(x)=(T+F)(x)$ as $nto infty$, also $y_nto y$ is clear by definition so $y=(T+F)x$ gives $overline{G_{T+F}}subset G_{T+F}$. Hence our claim is proved.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for a very detailed answer
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:30










          • $begingroup$
            welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
            $endgroup$
            – Sujit Bhattacharyya
            Dec 4 '18 at 11:32










          • $begingroup$
            Yeah it was meant to say $neq$, edited it now :)
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:35











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          $begingroup$

          Note that for any $lambdainmathbb{C}$, $D(T)=D(T-lambda I)$ and both of $T$ and $I$ are closed. Also, for any two closed operators $T, F$ their linear sum is also closed.



          For proof, you simply consider $T:D(T)to X$ and $F:D(F)to X$ be two closed operators then their corresponding graphs are closed i.e $G_T, G_F$ are closed. We now show $G_{T+F}$ is closed which will imply $T+F:D(T)cap D(F)to X$ defined by $(T+F)x=T(x)+F(x)$is closed.



          For this let $(x,y)in overline{G_{F+T}}$ then $exists$ a sequence ${(x_n,y_n)}_{ninmathbb{N}}$ in $G_{F+T}$ converging to $(x,y)$ with $y_n=(T+F)(x_n),forall ninmathbb{N}implies y_n=T(x_n)+F(x_n),forall ninmathbb{N}$.



          Since $G_T, G_F$ are both closed so $T(x_n)to T(x)$, $F(x_n)to F(x)$ as $x_nto x$ as $nto infty$. Hence, $y_n=T(x_n)+F(x_n)to T(x)+F(x)=(T+F)(x)$ as $nto infty$, also $y_nto y$ is clear by definition so $y=(T+F)x$ gives $overline{G_{T+F}}subset G_{T+F}$. Hence our claim is proved.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for a very detailed answer
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:30










          • $begingroup$
            welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
            $endgroup$
            – Sujit Bhattacharyya
            Dec 4 '18 at 11:32










          • $begingroup$
            Yeah it was meant to say $neq$, edited it now :)
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:35
















          1












          $begingroup$

          Note that for any $lambdainmathbb{C}$, $D(T)=D(T-lambda I)$ and both of $T$ and $I$ are closed. Also, for any two closed operators $T, F$ their linear sum is also closed.



          For proof, you simply consider $T:D(T)to X$ and $F:D(F)to X$ be two closed operators then their corresponding graphs are closed i.e $G_T, G_F$ are closed. We now show $G_{T+F}$ is closed which will imply $T+F:D(T)cap D(F)to X$ defined by $(T+F)x=T(x)+F(x)$is closed.



          For this let $(x,y)in overline{G_{F+T}}$ then $exists$ a sequence ${(x_n,y_n)}_{ninmathbb{N}}$ in $G_{F+T}$ converging to $(x,y)$ with $y_n=(T+F)(x_n),forall ninmathbb{N}implies y_n=T(x_n)+F(x_n),forall ninmathbb{N}$.



          Since $G_T, G_F$ are both closed so $T(x_n)to T(x)$, $F(x_n)to F(x)$ as $x_nto x$ as $nto infty$. Hence, $y_n=T(x_n)+F(x_n)to T(x)+F(x)=(T+F)(x)$ as $nto infty$, also $y_nto y$ is clear by definition so $y=(T+F)x$ gives $overline{G_{T+F}}subset G_{T+F}$. Hence our claim is proved.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for a very detailed answer
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:30










          • $begingroup$
            welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
            $endgroup$
            – Sujit Bhattacharyya
            Dec 4 '18 at 11:32










          • $begingroup$
            Yeah it was meant to say $neq$, edited it now :)
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:35














          1












          1








          1





          $begingroup$

          Note that for any $lambdainmathbb{C}$, $D(T)=D(T-lambda I)$ and both of $T$ and $I$ are closed. Also, for any two closed operators $T, F$ their linear sum is also closed.



          For proof, you simply consider $T:D(T)to X$ and $F:D(F)to X$ be two closed operators then their corresponding graphs are closed i.e $G_T, G_F$ are closed. We now show $G_{T+F}$ is closed which will imply $T+F:D(T)cap D(F)to X$ defined by $(T+F)x=T(x)+F(x)$is closed.



          For this let $(x,y)in overline{G_{F+T}}$ then $exists$ a sequence ${(x_n,y_n)}_{ninmathbb{N}}$ in $G_{F+T}$ converging to $(x,y)$ with $y_n=(T+F)(x_n),forall ninmathbb{N}implies y_n=T(x_n)+F(x_n),forall ninmathbb{N}$.



          Since $G_T, G_F$ are both closed so $T(x_n)to T(x)$, $F(x_n)to F(x)$ as $x_nto x$ as $nto infty$. Hence, $y_n=T(x_n)+F(x_n)to T(x)+F(x)=(T+F)(x)$ as $nto infty$, also $y_nto y$ is clear by definition so $y=(T+F)x$ gives $overline{G_{T+F}}subset G_{T+F}$. Hence our claim is proved.






          share|cite|improve this answer









          $endgroup$



          Note that for any $lambdainmathbb{C}$, $D(T)=D(T-lambda I)$ and both of $T$ and $I$ are closed. Also, for any two closed operators $T, F$ their linear sum is also closed.



          For proof, you simply consider $T:D(T)to X$ and $F:D(F)to X$ be two closed operators then their corresponding graphs are closed i.e $G_T, G_F$ are closed. We now show $G_{T+F}$ is closed which will imply $T+F:D(T)cap D(F)to X$ defined by $(T+F)x=T(x)+F(x)$is closed.



          For this let $(x,y)in overline{G_{F+T}}$ then $exists$ a sequence ${(x_n,y_n)}_{ninmathbb{N}}$ in $G_{F+T}$ converging to $(x,y)$ with $y_n=(T+F)(x_n),forall ninmathbb{N}implies y_n=T(x_n)+F(x_n),forall ninmathbb{N}$.



          Since $G_T, G_F$ are both closed so $T(x_n)to T(x)$, $F(x_n)to F(x)$ as $x_nto x$ as $nto infty$. Hence, $y_n=T(x_n)+F(x_n)to T(x)+F(x)=(T+F)(x)$ as $nto infty$, also $y_nto y$ is clear by definition so $y=(T+F)x$ gives $overline{G_{T+F}}subset G_{T+F}$. Hence our claim is proved.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 4 '18 at 11:28









          Sujit BhattacharyyaSujit Bhattacharyya

          1,092418




          1,092418












          • $begingroup$
            Thank you for a very detailed answer
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:30










          • $begingroup$
            welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
            $endgroup$
            – Sujit Bhattacharyya
            Dec 4 '18 at 11:32










          • $begingroup$
            Yeah it was meant to say $neq$, edited it now :)
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:35


















          • $begingroup$
            Thank you for a very detailed answer
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:30










          • $begingroup$
            welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
            $endgroup$
            – Sujit Bhattacharyya
            Dec 4 '18 at 11:32










          • $begingroup$
            Yeah it was meant to say $neq$, edited it now :)
            $endgroup$
            – thaumoctopus
            Dec 4 '18 at 11:35
















          $begingroup$
          Thank you for a very detailed answer
          $endgroup$
          – thaumoctopus
          Dec 4 '18 at 11:30




          $begingroup$
          Thank you for a very detailed answer
          $endgroup$
          – thaumoctopus
          Dec 4 '18 at 11:30












          $begingroup$
          welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
          $endgroup$
          – Sujit Bhattacharyya
          Dec 4 '18 at 11:32




          $begingroup$
          welcome. For your case, we have the domain set zero $D(T)subset X={0}$ so $D(T)={0}$ so the result holds trivially.
          $endgroup$
          – Sujit Bhattacharyya
          Dec 4 '18 at 11:32












          $begingroup$
          Yeah it was meant to say $neq$, edited it now :)
          $endgroup$
          – thaumoctopus
          Dec 4 '18 at 11:35




          $begingroup$
          Yeah it was meant to say $neq$, edited it now :)
          $endgroup$
          – thaumoctopus
          Dec 4 '18 at 11:35


















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