Find the min and max distance from origin of the curve $vert z+frac1zvert=a$

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$z$ is a complex number, by the way.



I've tried a lot of things and always end up with a huge algebraic mess and I've wondered if anyone of you has any idea on how to approach this problem.



One of the most "satisfactory" things I've done so far is to expand both the real and imaginary parts for the equation vert z + 1/z vert^2 = a^2. These parts yield the constraints:



$2xy(1+1/(x^2+y^2)^2)=0$ ;



$x^2-y^2+frac2(x^2-y^2)x^2+y^2+frac(x^2-y^2)(x^2+y^2)^2=a^2 $



I've graphed the equations and supposedly the intersection should be my set of points, but this method is far from analytical.



What can I do? Any hints on how to approach this?







share|cite|improve this question

























    up vote
    2
    down vote

    favorite












    $z$ is a complex number, by the way.



    I've tried a lot of things and always end up with a huge algebraic mess and I've wondered if anyone of you has any idea on how to approach this problem.



    One of the most "satisfactory" things I've done so far is to expand both the real and imaginary parts for the equation vert z + 1/z vert^2 = a^2. These parts yield the constraints:



    $2xy(1+1/(x^2+y^2)^2)=0$ ;



    $x^2-y^2+frac2(x^2-y^2)x^2+y^2+frac(x^2-y^2)(x^2+y^2)^2=a^2 $



    I've graphed the equations and supposedly the intersection should be my set of points, but this method is far from analytical.



    What can I do? Any hints on how to approach this?







    share|cite|improve this question























      up vote
      2
      down vote

      favorite









      up vote
      2
      down vote

      favorite











      $z$ is a complex number, by the way.



      I've tried a lot of things and always end up with a huge algebraic mess and I've wondered if anyone of you has any idea on how to approach this problem.



      One of the most "satisfactory" things I've done so far is to expand both the real and imaginary parts for the equation vert z + 1/z vert^2 = a^2. These parts yield the constraints:



      $2xy(1+1/(x^2+y^2)^2)=0$ ;



      $x^2-y^2+frac2(x^2-y^2)x^2+y^2+frac(x^2-y^2)(x^2+y^2)^2=a^2 $



      I've graphed the equations and supposedly the intersection should be my set of points, but this method is far from analytical.



      What can I do? Any hints on how to approach this?







      share|cite|improve this question













      $z$ is a complex number, by the way.



      I've tried a lot of things and always end up with a huge algebraic mess and I've wondered if anyone of you has any idea on how to approach this problem.



      One of the most "satisfactory" things I've done so far is to expand both the real and imaginary parts for the equation vert z + 1/z vert^2 = a^2. These parts yield the constraints:



      $2xy(1+1/(x^2+y^2)^2)=0$ ;



      $x^2-y^2+frac2(x^2-y^2)x^2+y^2+frac(x^2-y^2)(x^2+y^2)^2=a^2 $



      I've graphed the equations and supposedly the intersection should be my set of points, but this method is far from analytical.



      What can I do? Any hints on how to approach this?









      share|cite|improve this question












      share|cite|improve this question




      share|cite|improve this question








      edited Aug 3 at 16:36









      Batominovski

      22.6k22776




      22.6k22776









      asked Aug 23 '15 at 1:40









      DLV

      767820




      767820




















          1 Answer
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          up vote
          4
          down vote



          accepted










          Short Solution:



          By the Triangle Inequality,
          $$a=left|z+frac1zright|geq left||z|-frac1zright|,,$$
          giving
          $$-aleq |z|-frac1zleq +a,,$$
          which means
          $$fracsqrta^2+4-a2 leq |z| leq fracsqrta^2+4+a2,.$$
          Since $z=pm fracsqrta^2+4+a2$ and $z=pmfracsqrta^2+4-a2$ are solutions, both bounds are sharp.




          Long Solution: I like this one better, though, because it tells you all possible values of $|z|$.



          Suppose that $z=rexp(textitheta)$ with $r>0$ and $thetainmathbbR$ is a solution to $left|z+frac1zright|=a$. Then, $$a^2=left|z+frac1zright|^2=r^2+frac1r^2+2cos(2theta),.$$
          Hence,
          $$a^2-2leq r^2+frac1r^2 leq a^2+2,.tag*$$



          If $a>2$, then, $r$ must satisfy $left(r+frac1rright)^2geq a^2$, or $r+frac1rgeq a$, as well as$left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. This means
          $$0leq rleq fraca-sqrta^2-42text or rgeq fraca+sqrta^2-42,,$$ along with $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$
          Observe that $fraca-sqrta^2-42> fracsqrta^2+4-a2$ for all $a>2$. Thence,
          $$fracsqrta^2+4-a2leq r leq fraca-sqrta^2-42text or fraca+sqrta^2-42leq rleq fracsqrta^2+4+a2,.$$



          If $0 leq aleq 2$, then we have by AM-GM that $ r^2+frac1r^2geq 2geq a^2-2$. Thus, only the inequality on the right-hand side of (*) is relevant. We have $left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. Therefore, $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$



          Consequently, in both cases, we have $fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2$. The equality on the left side is satisfied when $z=pmfracsqrta^2+4-a2$, and the equality on the right is satisfied iff $z=pmfracsqrta^2+4+a2$. Ergo, the maximum value of $|z|=r$ is $fracsqrta^2+4+a2$, whereas the minimum value is $fracsqrta^2+4-a2$.






          share|cite|improve this answer























          • Thanks. I'm studying your answer, I'll provide feedback a bit later.
            – DLV
            Aug 23 '15 at 2:44










          • I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
            – DLV
            Aug 23 '15 at 3:04







          • 1




            If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
            – Batominovski
            Aug 23 '15 at 3:10







          • 1




            I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
            – Batominovski
            Aug 23 '15 at 3:19











          • Thanks! I'll keep studying your answer but I think I've got it now :D
            – DLV
            Aug 23 '15 at 3:46










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          1 Answer
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          1 Answer
          1






          active

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          active

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          active

          oldest

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          up vote
          4
          down vote



          accepted










          Short Solution:



          By the Triangle Inequality,
          $$a=left|z+frac1zright|geq left||z|-frac1zright|,,$$
          giving
          $$-aleq |z|-frac1zleq +a,,$$
          which means
          $$fracsqrta^2+4-a2 leq |z| leq fracsqrta^2+4+a2,.$$
          Since $z=pm fracsqrta^2+4+a2$ and $z=pmfracsqrta^2+4-a2$ are solutions, both bounds are sharp.




          Long Solution: I like this one better, though, because it tells you all possible values of $|z|$.



          Suppose that $z=rexp(textitheta)$ with $r>0$ and $thetainmathbbR$ is a solution to $left|z+frac1zright|=a$. Then, $$a^2=left|z+frac1zright|^2=r^2+frac1r^2+2cos(2theta),.$$
          Hence,
          $$a^2-2leq r^2+frac1r^2 leq a^2+2,.tag*$$



          If $a>2$, then, $r$ must satisfy $left(r+frac1rright)^2geq a^2$, or $r+frac1rgeq a$, as well as$left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. This means
          $$0leq rleq fraca-sqrta^2-42text or rgeq fraca+sqrta^2-42,,$$ along with $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$
          Observe that $fraca-sqrta^2-42> fracsqrta^2+4-a2$ for all $a>2$. Thence,
          $$fracsqrta^2+4-a2leq r leq fraca-sqrta^2-42text or fraca+sqrta^2-42leq rleq fracsqrta^2+4+a2,.$$



          If $0 leq aleq 2$, then we have by AM-GM that $ r^2+frac1r^2geq 2geq a^2-2$. Thus, only the inequality on the right-hand side of (*) is relevant. We have $left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. Therefore, $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$



          Consequently, in both cases, we have $fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2$. The equality on the left side is satisfied when $z=pmfracsqrta^2+4-a2$, and the equality on the right is satisfied iff $z=pmfracsqrta^2+4+a2$. Ergo, the maximum value of $|z|=r$ is $fracsqrta^2+4+a2$, whereas the minimum value is $fracsqrta^2+4-a2$.






          share|cite|improve this answer























          • Thanks. I'm studying your answer, I'll provide feedback a bit later.
            – DLV
            Aug 23 '15 at 2:44










          • I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
            – DLV
            Aug 23 '15 at 3:04







          • 1




            If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
            – Batominovski
            Aug 23 '15 at 3:10







          • 1




            I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
            – Batominovski
            Aug 23 '15 at 3:19











          • Thanks! I'll keep studying your answer but I think I've got it now :D
            – DLV
            Aug 23 '15 at 3:46














          up vote
          4
          down vote



          accepted










          Short Solution:



          By the Triangle Inequality,
          $$a=left|z+frac1zright|geq left||z|-frac1zright|,,$$
          giving
          $$-aleq |z|-frac1zleq +a,,$$
          which means
          $$fracsqrta^2+4-a2 leq |z| leq fracsqrta^2+4+a2,.$$
          Since $z=pm fracsqrta^2+4+a2$ and $z=pmfracsqrta^2+4-a2$ are solutions, both bounds are sharp.




          Long Solution: I like this one better, though, because it tells you all possible values of $|z|$.



          Suppose that $z=rexp(textitheta)$ with $r>0$ and $thetainmathbbR$ is a solution to $left|z+frac1zright|=a$. Then, $$a^2=left|z+frac1zright|^2=r^2+frac1r^2+2cos(2theta),.$$
          Hence,
          $$a^2-2leq r^2+frac1r^2 leq a^2+2,.tag*$$



          If $a>2$, then, $r$ must satisfy $left(r+frac1rright)^2geq a^2$, or $r+frac1rgeq a$, as well as$left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. This means
          $$0leq rleq fraca-sqrta^2-42text or rgeq fraca+sqrta^2-42,,$$ along with $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$
          Observe that $fraca-sqrta^2-42> fracsqrta^2+4-a2$ for all $a>2$. Thence,
          $$fracsqrta^2+4-a2leq r leq fraca-sqrta^2-42text or fraca+sqrta^2-42leq rleq fracsqrta^2+4+a2,.$$



          If $0 leq aleq 2$, then we have by AM-GM that $ r^2+frac1r^2geq 2geq a^2-2$. Thus, only the inequality on the right-hand side of (*) is relevant. We have $left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. Therefore, $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$



          Consequently, in both cases, we have $fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2$. The equality on the left side is satisfied when $z=pmfracsqrta^2+4-a2$, and the equality on the right is satisfied iff $z=pmfracsqrta^2+4+a2$. Ergo, the maximum value of $|z|=r$ is $fracsqrta^2+4+a2$, whereas the minimum value is $fracsqrta^2+4-a2$.






          share|cite|improve this answer























          • Thanks. I'm studying your answer, I'll provide feedback a bit later.
            – DLV
            Aug 23 '15 at 2:44










          • I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
            – DLV
            Aug 23 '15 at 3:04







          • 1




            If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
            – Batominovski
            Aug 23 '15 at 3:10







          • 1




            I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
            – Batominovski
            Aug 23 '15 at 3:19











          • Thanks! I'll keep studying your answer but I think I've got it now :D
            – DLV
            Aug 23 '15 at 3:46












          up vote
          4
          down vote



          accepted







          up vote
          4
          down vote



          accepted






          Short Solution:



          By the Triangle Inequality,
          $$a=left|z+frac1zright|geq left||z|-frac1zright|,,$$
          giving
          $$-aleq |z|-frac1zleq +a,,$$
          which means
          $$fracsqrta^2+4-a2 leq |z| leq fracsqrta^2+4+a2,.$$
          Since $z=pm fracsqrta^2+4+a2$ and $z=pmfracsqrta^2+4-a2$ are solutions, both bounds are sharp.




          Long Solution: I like this one better, though, because it tells you all possible values of $|z|$.



          Suppose that $z=rexp(textitheta)$ with $r>0$ and $thetainmathbbR$ is a solution to $left|z+frac1zright|=a$. Then, $$a^2=left|z+frac1zright|^2=r^2+frac1r^2+2cos(2theta),.$$
          Hence,
          $$a^2-2leq r^2+frac1r^2 leq a^2+2,.tag*$$



          If $a>2$, then, $r$ must satisfy $left(r+frac1rright)^2geq a^2$, or $r+frac1rgeq a$, as well as$left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. This means
          $$0leq rleq fraca-sqrta^2-42text or rgeq fraca+sqrta^2-42,,$$ along with $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$
          Observe that $fraca-sqrta^2-42> fracsqrta^2+4-a2$ for all $a>2$. Thence,
          $$fracsqrta^2+4-a2leq r leq fraca-sqrta^2-42text or fraca+sqrta^2-42leq rleq fracsqrta^2+4+a2,.$$



          If $0 leq aleq 2$, then we have by AM-GM that $ r^2+frac1r^2geq 2geq a^2-2$. Thus, only the inequality on the right-hand side of (*) is relevant. We have $left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. Therefore, $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$



          Consequently, in both cases, we have $fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2$. The equality on the left side is satisfied when $z=pmfracsqrta^2+4-a2$, and the equality on the right is satisfied iff $z=pmfracsqrta^2+4+a2$. Ergo, the maximum value of $|z|=r$ is $fracsqrta^2+4+a2$, whereas the minimum value is $fracsqrta^2+4-a2$.






          share|cite|improve this answer















          Short Solution:



          By the Triangle Inequality,
          $$a=left|z+frac1zright|geq left||z|-frac1zright|,,$$
          giving
          $$-aleq |z|-frac1zleq +a,,$$
          which means
          $$fracsqrta^2+4-a2 leq |z| leq fracsqrta^2+4+a2,.$$
          Since $z=pm fracsqrta^2+4+a2$ and $z=pmfracsqrta^2+4-a2$ are solutions, both bounds are sharp.




          Long Solution: I like this one better, though, because it tells you all possible values of $|z|$.



          Suppose that $z=rexp(textitheta)$ with $r>0$ and $thetainmathbbR$ is a solution to $left|z+frac1zright|=a$. Then, $$a^2=left|z+frac1zright|^2=r^2+frac1r^2+2cos(2theta),.$$
          Hence,
          $$a^2-2leq r^2+frac1r^2 leq a^2+2,.tag*$$



          If $a>2$, then, $r$ must satisfy $left(r+frac1rright)^2geq a^2$, or $r+frac1rgeq a$, as well as$left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. This means
          $$0leq rleq fraca-sqrta^2-42text or rgeq fraca+sqrta^2-42,,$$ along with $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$
          Observe that $fraca-sqrta^2-42> fracsqrta^2+4-a2$ for all $a>2$. Thence,
          $$fracsqrta^2+4-a2leq r leq fraca-sqrta^2-42text or fraca+sqrta^2-42leq rleq fracsqrta^2+4+a2,.$$



          If $0 leq aleq 2$, then we have by AM-GM that $ r^2+frac1r^2geq 2geq a^2-2$. Thus, only the inequality on the right-hand side of (*) is relevant. We have $left(r-frac1rright)^2leq a^2$, or $-aleq r-frac1rleq +a$. Therefore, $$fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2,.$$



          Consequently, in both cases, we have $fracsqrta^2+4-a2 leq r leq fracsqrta^2+4+a2$. The equality on the left side is satisfied when $z=pmfracsqrta^2+4-a2$, and the equality on the right is satisfied iff $z=pmfracsqrta^2+4+a2$. Ergo, the maximum value of $|z|=r$ is $fracsqrta^2+4+a2$, whereas the minimum value is $fracsqrta^2+4-a2$.







          share|cite|improve this answer















          share|cite|improve this answer



          share|cite|improve this answer








          edited May 25 '16 at 15:13


























          answered Aug 23 '15 at 2:32









          Batominovski

          22.6k22776




          22.6k22776











          • Thanks. I'm studying your answer, I'll provide feedback a bit later.
            – DLV
            Aug 23 '15 at 2:44










          • I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
            – DLV
            Aug 23 '15 at 3:04







          • 1




            If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
            – Batominovski
            Aug 23 '15 at 3:10







          • 1




            I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
            – Batominovski
            Aug 23 '15 at 3:19











          • Thanks! I'll keep studying your answer but I think I've got it now :D
            – DLV
            Aug 23 '15 at 3:46
















          • Thanks. I'm studying your answer, I'll provide feedback a bit later.
            – DLV
            Aug 23 '15 at 2:44










          • I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
            – DLV
            Aug 23 '15 at 3:04







          • 1




            If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
            – Batominovski
            Aug 23 '15 at 3:10







          • 1




            I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
            – Batominovski
            Aug 23 '15 at 3:19











          • Thanks! I'll keep studying your answer but I think I've got it now :D
            – DLV
            Aug 23 '15 at 3:46















          Thanks. I'm studying your answer, I'll provide feedback a bit later.
          – DLV
          Aug 23 '15 at 2:44




          Thanks. I'm studying your answer, I'll provide feedback a bit later.
          – DLV
          Aug 23 '15 at 2:44












          I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
          – DLV
          Aug 23 '15 at 3:04





          I've stared at the equations from paragraph 3 onwards for a long time now. I don't know where they're coming from... What should I try to see where they're coming from? By paragraph 3 I mean after the conclusion that $-a leq r-1/r leq a$
          – DLV
          Aug 23 '15 at 3:04





          1




          1




          If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
          – Batominovski
          Aug 23 '15 at 3:10





          If $x^2leq y^2$ and $ygeq 0$, then $|x|leq |y|=y$, so $-yleq xleq +y$. I have posted another solution that is a lot simpler.
          – Batominovski
          Aug 23 '15 at 3:10





          1




          1




          I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
          – Batominovski
          Aug 23 '15 at 3:19





          I think I misunderstood your question. If $r-frac1rleq +a$ and $rgeq 0$, then you have $0 leq r leq fracsqrta^2+4+a2$, since it is equivalent to $r^2-2ra-1leq 0$. If $r-frac1rgeq-a$ and $rgeq 0$, then you have $rgeq fracsqrta^2+4-a2$, since it is equivalent to $r^2+2ra-1geq 0$.
          – Batominovski
          Aug 23 '15 at 3:19













          Thanks! I'll keep studying your answer but I think I've got it now :D
          – DLV
          Aug 23 '15 at 3:46




          Thanks! I'll keep studying your answer but I think I've got it now :D
          – DLV
          Aug 23 '15 at 3:46












           

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