Solutions check. Find $int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt$












0














Let $x > 0$. Find $$lim_{b to infty} int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt$$



I solved this problem but I am not quite confident about my steps. So please, could you check my solutions and tell me if there is something wrong??. Thanks in advanced!



First approuch:



Let's just focus on the first member of the integral:
$$int_{-b}^{b} frac{1}{t + ix} dt =
int_{-b}^{b} frac{1}{t + ix} Big( frac{t - ix}{t - ix} Big) dt =
int_{-b}^{b} frac{t - ix}{t^2 + x^2} dt$$



where the integrand can be again separated:



$$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt - int_{-b}^{b} frac{ix}{t^2 + x^2}dt$$



Then, since the integral is about a real number $t$, $i$ can be treated as a constant. Now, I notice that the second integrand is an even function so the integral from $-b$ to $b$ is $0$. So we have:



$$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt =
frac{1}{2}lnBig(t^2 + x^2 Big) Bigrvert_{-b}^{b} =
frac{1}{2}lnBig(frac{b^2 + x^2}{b^2 + x^2} Big) = 0$$



Following almost the same steps one can also show that $$int_{-b}^{b} frac{1}{t - ix} dt= 0$$



So the question reminds as $$lim_{b to infty} (0 + 0) = 0$$



Is this correct?? I mean:




  • Can $i$ be really treated as a constat in an integral which is about a real number??


  • Why would the question ask for about a limit int the infinite if the integral was destined to be $0$??



Second approuch



If $i$ can be really treated as a constant then the integral of the function $frac{1}{t + ix}$ would be $ln(t + ix)$, since $ix$ would be also a constant, so we have:



$$int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt=
ln(t + ix)Bigrvert_{-b}^{b} - ln(t - ix)Bigrvert_{-b}^{b}$$



Since $ix$ is just a constant then $ln$ fullfills the same properties as the well-known real valued function $ln$, so:



$$ = lnBig(frac{b + ix}{-b + ix} Big) - lnBig(frac{b - ix}{-b - ix} Big) =
lnBig(frac{frac{b + ix}{-b + ix}}{frac{b - ix}{-b - ix}} Big) = ln(1) = 0$$



Soo the answer would be $lim_{b to infty} 0 = 0$. I am really confused with theese steps since $ln$ function was evaluated in $b + ix$ and although I know that there is a counter-part complex version of the real function $ln$, I had accepted $ix$ as a costant, so there wouldn't be nothing wrong, right??



Third approuch



If $i$ can be really treated as a constant, then:



$$int_{-b}^{b} Big( frac{1}{t+ ix} - frac{1}{t - ix} Big) dt =
int_{-b}^{b} Big( frac{-2ix}{t^2 + x^2} Big) dt$$



because the integrand is an even function, that integral is $0$. So we have again $lim_{b to infty} 0 = 0$



Any comment about what I wrote would be really appreciated!










share|cite|improve this question





























    0














    Let $x > 0$. Find $$lim_{b to infty} int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt$$



    I solved this problem but I am not quite confident about my steps. So please, could you check my solutions and tell me if there is something wrong??. Thanks in advanced!



    First approuch:



    Let's just focus on the first member of the integral:
    $$int_{-b}^{b} frac{1}{t + ix} dt =
    int_{-b}^{b} frac{1}{t + ix} Big( frac{t - ix}{t - ix} Big) dt =
    int_{-b}^{b} frac{t - ix}{t^2 + x^2} dt$$



    where the integrand can be again separated:



    $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt - int_{-b}^{b} frac{ix}{t^2 + x^2}dt$$



    Then, since the integral is about a real number $t$, $i$ can be treated as a constant. Now, I notice that the second integrand is an even function so the integral from $-b$ to $b$ is $0$. So we have:



    $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt =
    frac{1}{2}lnBig(t^2 + x^2 Big) Bigrvert_{-b}^{b} =
    frac{1}{2}lnBig(frac{b^2 + x^2}{b^2 + x^2} Big) = 0$$



    Following almost the same steps one can also show that $$int_{-b}^{b} frac{1}{t - ix} dt= 0$$



    So the question reminds as $$lim_{b to infty} (0 + 0) = 0$$



    Is this correct?? I mean:




    • Can $i$ be really treated as a constat in an integral which is about a real number??


    • Why would the question ask for about a limit int the infinite if the integral was destined to be $0$??



    Second approuch



    If $i$ can be really treated as a constant then the integral of the function $frac{1}{t + ix}$ would be $ln(t + ix)$, since $ix$ would be also a constant, so we have:



    $$int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt=
    ln(t + ix)Bigrvert_{-b}^{b} - ln(t - ix)Bigrvert_{-b}^{b}$$



    Since $ix$ is just a constant then $ln$ fullfills the same properties as the well-known real valued function $ln$, so:



    $$ = lnBig(frac{b + ix}{-b + ix} Big) - lnBig(frac{b - ix}{-b - ix} Big) =
    lnBig(frac{frac{b + ix}{-b + ix}}{frac{b - ix}{-b - ix}} Big) = ln(1) = 0$$



    Soo the answer would be $lim_{b to infty} 0 = 0$. I am really confused with theese steps since $ln$ function was evaluated in $b + ix$ and although I know that there is a counter-part complex version of the real function $ln$, I had accepted $ix$ as a costant, so there wouldn't be nothing wrong, right??



    Third approuch



    If $i$ can be really treated as a constant, then:



    $$int_{-b}^{b} Big( frac{1}{t+ ix} - frac{1}{t - ix} Big) dt =
    int_{-b}^{b} Big( frac{-2ix}{t^2 + x^2} Big) dt$$



    because the integrand is an even function, that integral is $0$. So we have again $lim_{b to infty} 0 = 0$



    Any comment about what I wrote would be really appreciated!










    share|cite|improve this question



























      0












      0








      0







      Let $x > 0$. Find $$lim_{b to infty} int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt$$



      I solved this problem but I am not quite confident about my steps. So please, could you check my solutions and tell me if there is something wrong??. Thanks in advanced!



      First approuch:



      Let's just focus on the first member of the integral:
      $$int_{-b}^{b} frac{1}{t + ix} dt =
      int_{-b}^{b} frac{1}{t + ix} Big( frac{t - ix}{t - ix} Big) dt =
      int_{-b}^{b} frac{t - ix}{t^2 + x^2} dt$$



      where the integrand can be again separated:



      $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt - int_{-b}^{b} frac{ix}{t^2 + x^2}dt$$



      Then, since the integral is about a real number $t$, $i$ can be treated as a constant. Now, I notice that the second integrand is an even function so the integral from $-b$ to $b$ is $0$. So we have:



      $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt =
      frac{1}{2}lnBig(t^2 + x^2 Big) Bigrvert_{-b}^{b} =
      frac{1}{2}lnBig(frac{b^2 + x^2}{b^2 + x^2} Big) = 0$$



      Following almost the same steps one can also show that $$int_{-b}^{b} frac{1}{t - ix} dt= 0$$



      So the question reminds as $$lim_{b to infty} (0 + 0) = 0$$



      Is this correct?? I mean:




      • Can $i$ be really treated as a constat in an integral which is about a real number??


      • Why would the question ask for about a limit int the infinite if the integral was destined to be $0$??



      Second approuch



      If $i$ can be really treated as a constant then the integral of the function $frac{1}{t + ix}$ would be $ln(t + ix)$, since $ix$ would be also a constant, so we have:



      $$int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt=
      ln(t + ix)Bigrvert_{-b}^{b} - ln(t - ix)Bigrvert_{-b}^{b}$$



      Since $ix$ is just a constant then $ln$ fullfills the same properties as the well-known real valued function $ln$, so:



      $$ = lnBig(frac{b + ix}{-b + ix} Big) - lnBig(frac{b - ix}{-b - ix} Big) =
      lnBig(frac{frac{b + ix}{-b + ix}}{frac{b - ix}{-b - ix}} Big) = ln(1) = 0$$



      Soo the answer would be $lim_{b to infty} 0 = 0$. I am really confused with theese steps since $ln$ function was evaluated in $b + ix$ and although I know that there is a counter-part complex version of the real function $ln$, I had accepted $ix$ as a costant, so there wouldn't be nothing wrong, right??



      Third approuch



      If $i$ can be really treated as a constant, then:



      $$int_{-b}^{b} Big( frac{1}{t+ ix} - frac{1}{t - ix} Big) dt =
      int_{-b}^{b} Big( frac{-2ix}{t^2 + x^2} Big) dt$$



      because the integrand is an even function, that integral is $0$. So we have again $lim_{b to infty} 0 = 0$



      Any comment about what I wrote would be really appreciated!










      share|cite|improve this question















      Let $x > 0$. Find $$lim_{b to infty} int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt$$



      I solved this problem but I am not quite confident about my steps. So please, could you check my solutions and tell me if there is something wrong??. Thanks in advanced!



      First approuch:



      Let's just focus on the first member of the integral:
      $$int_{-b}^{b} frac{1}{t + ix} dt =
      int_{-b}^{b} frac{1}{t + ix} Big( frac{t - ix}{t - ix} Big) dt =
      int_{-b}^{b} frac{t - ix}{t^2 + x^2} dt$$



      where the integrand can be again separated:



      $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt - int_{-b}^{b} frac{ix}{t^2 + x^2}dt$$



      Then, since the integral is about a real number $t$, $i$ can be treated as a constant. Now, I notice that the second integrand is an even function so the integral from $-b$ to $b$ is $0$. So we have:



      $$ = int_{-b}^{b} frac{t}{t^2 + x^2}dt =
      frac{1}{2}lnBig(t^2 + x^2 Big) Bigrvert_{-b}^{b} =
      frac{1}{2}lnBig(frac{b^2 + x^2}{b^2 + x^2} Big) = 0$$



      Following almost the same steps one can also show that $$int_{-b}^{b} frac{1}{t - ix} dt= 0$$



      So the question reminds as $$lim_{b to infty} (0 + 0) = 0$$



      Is this correct?? I mean:




      • Can $i$ be really treated as a constat in an integral which is about a real number??


      • Why would the question ask for about a limit int the infinite if the integral was destined to be $0$??



      Second approuch



      If $i$ can be really treated as a constant then the integral of the function $frac{1}{t + ix}$ would be $ln(t + ix)$, since $ix$ would be also a constant, so we have:



      $$int_{-b}^{b} Big( frac{1}{t + ix} - frac{1}{t - ix} Big) dt=
      ln(t + ix)Bigrvert_{-b}^{b} - ln(t - ix)Bigrvert_{-b}^{b}$$



      Since $ix$ is just a constant then $ln$ fullfills the same properties as the well-known real valued function $ln$, so:



      $$ = lnBig(frac{b + ix}{-b + ix} Big) - lnBig(frac{b - ix}{-b - ix} Big) =
      lnBig(frac{frac{b + ix}{-b + ix}}{frac{b - ix}{-b - ix}} Big) = ln(1) = 0$$



      Soo the answer would be $lim_{b to infty} 0 = 0$. I am really confused with theese steps since $ln$ function was evaluated in $b + ix$ and although I know that there is a counter-part complex version of the real function $ln$, I had accepted $ix$ as a costant, so there wouldn't be nothing wrong, right??



      Third approuch



      If $i$ can be really treated as a constant, then:



      $$int_{-b}^{b} Big( frac{1}{t+ ix} - frac{1}{t - ix} Big) dt =
      int_{-b}^{b} Big( frac{-2ix}{t^2 + x^2} Big) dt$$



      because the integrand is an even function, that integral is $0$. So we have again $lim_{b to infty} 0 = 0$



      Any comment about what I wrote would be really appreciated!







      complex-analysis proof-verification contour-integration






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      edited Nov 21 '18 at 11:21









      Tianlalu

      3,09621038




      3,09621038










      asked Nov 21 '18 at 11:15









      Aldebaran

      284




      284






















          2 Answers
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          The problem with your first and third approaches is that you wrote that any even function $f(x)$, we get that $int_{-b}^b f(x) dx=0$. This statement is true for odd functions. For even functions, $int_{-b}^b f(x) dx=2int_0^b f(x) dx$.



          The problem with your second approach is that in the world of complex functions, logarithms are multiple-valued, and as such give you an answer that is true up to a $+2kpi i, kinmathbb{Z}$.






          share|cite|improve this answer





























            1














            If $x=0$, then the integral vanishes. Assume that $xne 0$. Then for $x>0$
            $$
            int_{-b}^bleft(frac{1}{t+ix}-frac{1}{t-ix}right),dt=
            int_{-b}^bfrac{-2ix,dt}{t^2+x^2}=int_{-b}^bfrac{-2i,d(t/x)}{(t/x)^2+1}
            =-2iint_{-b/x}^{b/x}frac{ds}{s^2+1}=-2i big(tan^{-1}(b/x)-tan^{-1}(-b/x)big)=-4itan^{-1}(b/x)to -4icdotfrac{pi}{2}=-2pi i,
            $$

            For $x<0$, the limit is $2pi i$.






            share|cite|improve this answer





















            • wao. thank you so much, but where was I wrong to get a different answer¡?¡?
              – Aldebaran
              Nov 21 '18 at 12:03











            Your Answer





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            2 Answers
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            active

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            2 Answers
            2






            active

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            active

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            active

            oldest

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            The problem with your first and third approaches is that you wrote that any even function $f(x)$, we get that $int_{-b}^b f(x) dx=0$. This statement is true for odd functions. For even functions, $int_{-b}^b f(x) dx=2int_0^b f(x) dx$.



            The problem with your second approach is that in the world of complex functions, logarithms are multiple-valued, and as such give you an answer that is true up to a $+2kpi i, kinmathbb{Z}$.






            share|cite|improve this answer


























              2














              The problem with your first and third approaches is that you wrote that any even function $f(x)$, we get that $int_{-b}^b f(x) dx=0$. This statement is true for odd functions. For even functions, $int_{-b}^b f(x) dx=2int_0^b f(x) dx$.



              The problem with your second approach is that in the world of complex functions, logarithms are multiple-valued, and as such give you an answer that is true up to a $+2kpi i, kinmathbb{Z}$.






              share|cite|improve this answer
























                2












                2








                2






                The problem with your first and third approaches is that you wrote that any even function $f(x)$, we get that $int_{-b}^b f(x) dx=0$. This statement is true for odd functions. For even functions, $int_{-b}^b f(x) dx=2int_0^b f(x) dx$.



                The problem with your second approach is that in the world of complex functions, logarithms are multiple-valued, and as such give you an answer that is true up to a $+2kpi i, kinmathbb{Z}$.






                share|cite|improve this answer












                The problem with your first and third approaches is that you wrote that any even function $f(x)$, we get that $int_{-b}^b f(x) dx=0$. This statement is true for odd functions. For even functions, $int_{-b}^b f(x) dx=2int_0^b f(x) dx$.



                The problem with your second approach is that in the world of complex functions, logarithms are multiple-valued, and as such give you an answer that is true up to a $+2kpi i, kinmathbb{Z}$.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Nov 21 '18 at 12:31









                MoKo19

                1914




                1914























                    1














                    If $x=0$, then the integral vanishes. Assume that $xne 0$. Then for $x>0$
                    $$
                    int_{-b}^bleft(frac{1}{t+ix}-frac{1}{t-ix}right),dt=
                    int_{-b}^bfrac{-2ix,dt}{t^2+x^2}=int_{-b}^bfrac{-2i,d(t/x)}{(t/x)^2+1}
                    =-2iint_{-b/x}^{b/x}frac{ds}{s^2+1}=-2i big(tan^{-1}(b/x)-tan^{-1}(-b/x)big)=-4itan^{-1}(b/x)to -4icdotfrac{pi}{2}=-2pi i,
                    $$

                    For $x<0$, the limit is $2pi i$.






                    share|cite|improve this answer





















                    • wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                      – Aldebaran
                      Nov 21 '18 at 12:03
















                    1














                    If $x=0$, then the integral vanishes. Assume that $xne 0$. Then for $x>0$
                    $$
                    int_{-b}^bleft(frac{1}{t+ix}-frac{1}{t-ix}right),dt=
                    int_{-b}^bfrac{-2ix,dt}{t^2+x^2}=int_{-b}^bfrac{-2i,d(t/x)}{(t/x)^2+1}
                    =-2iint_{-b/x}^{b/x}frac{ds}{s^2+1}=-2i big(tan^{-1}(b/x)-tan^{-1}(-b/x)big)=-4itan^{-1}(b/x)to -4icdotfrac{pi}{2}=-2pi i,
                    $$

                    For $x<0$, the limit is $2pi i$.






                    share|cite|improve this answer





















                    • wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                      – Aldebaran
                      Nov 21 '18 at 12:03














                    1












                    1








                    1






                    If $x=0$, then the integral vanishes. Assume that $xne 0$. Then for $x>0$
                    $$
                    int_{-b}^bleft(frac{1}{t+ix}-frac{1}{t-ix}right),dt=
                    int_{-b}^bfrac{-2ix,dt}{t^2+x^2}=int_{-b}^bfrac{-2i,d(t/x)}{(t/x)^2+1}
                    =-2iint_{-b/x}^{b/x}frac{ds}{s^2+1}=-2i big(tan^{-1}(b/x)-tan^{-1}(-b/x)big)=-4itan^{-1}(b/x)to -4icdotfrac{pi}{2}=-2pi i,
                    $$

                    For $x<0$, the limit is $2pi i$.






                    share|cite|improve this answer












                    If $x=0$, then the integral vanishes. Assume that $xne 0$. Then for $x>0$
                    $$
                    int_{-b}^bleft(frac{1}{t+ix}-frac{1}{t-ix}right),dt=
                    int_{-b}^bfrac{-2ix,dt}{t^2+x^2}=int_{-b}^bfrac{-2i,d(t/x)}{(t/x)^2+1}
                    =-2iint_{-b/x}^{b/x}frac{ds}{s^2+1}=-2i big(tan^{-1}(b/x)-tan^{-1}(-b/x)big)=-4itan^{-1}(b/x)to -4icdotfrac{pi}{2}=-2pi i,
                    $$

                    For $x<0$, the limit is $2pi i$.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Nov 21 '18 at 11:33









                    Yiorgos S. Smyrlis

                    62.8k1383163




                    62.8k1383163












                    • wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                      – Aldebaran
                      Nov 21 '18 at 12:03


















                    • wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                      – Aldebaran
                      Nov 21 '18 at 12:03
















                    wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                    – Aldebaran
                    Nov 21 '18 at 12:03




                    wao. thank you so much, but where was I wrong to get a different answer¡?¡?
                    – Aldebaran
                    Nov 21 '18 at 12:03


















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