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Homework Help: Solving Inequality With Complex Numbers Question

  1. Sep 16, 2011 #1
    "Solving Inequality With Complex Numbers" Question

    1. The problem statement, all variables and given/known data

    What does the inequality pz + conjugate(pz) + c < 0 represent if |p|^2 >c ?

    2. Relevant equations

    p is a constant and a member of the set of complex numbers. c is a constant and a member of the set of real numbers.

    3. The attempt at a solution

    First, for the EQUALITY pz + conjugate(pz) + c = 0, where p = a+ib, z = x+iy :

    pz + conjugate(pz) + c = 0 => (a+ib)(x+iy) + conjugate((a+ib)(x+iy)) + c = 0 => ax + iay + ibx - by + ax - iay -ixb -by + c = 0 => y=((a/b)*x)+ c/(2*b).

    So, the inequality is y> ((a/b)*x)+ c/(2*b).

    MY QUESTION IS: WHAT IS THE IMPORTANCE OF --> |p|^2 >c WHEN SHOWING THE INEQUALITY --> pz + conjugate(pz) + c < 0.

    I know that |p| = sqrt((a^2) + (b^2)), so |p|^2 >c implies ((a^2) + (b^2)) > c but I don't understand how this helps to show the the inequality pz + conjugate(pz) + c < 0... or more so, the inequality y > ((a/b)*x)+ c/(2*b).

    ALSO, if I were to graph this inequality, would it be a (diagonal) dotted line (I say dotted because points on the line are not a part of the solution) where the graph is shaded above the dotted line?

    Any help would be greatly appreciated.
    Thank you.
     
  2. jcsd
  3. Sep 16, 2011 #2

    HallsofIvy

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    Re: "Solving Inequality With Complex Numbers" Question

    It doesn't represent anything because the complex numbers do NOT form an "ordered field". There is no way to define "<" for complex numbers consistent with their arithmetic.

     
  4. Sep 16, 2011 #3

    I like Serena

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    Re: "Solving Inequality With Complex Numbers" Question

    That doesn't seem to be quite right, since the resulting expression is a real number.
    And for real numbers the "<" inequality is defined.

    pz + conjugate(pz) = 2 Re(pz)
     
  5. Sep 16, 2011 #4

    I like Serena

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    I don't see any particular relevance either to |p|2 > c.

    Not quite.
    The direction of the inequality depends on the sign of b.
    If b is negative, the inequality is inverted.


    And I agree that you get a shaded half surface bounded by your dotted line.
    You only have to be careful, which half surface you have exactly.
     
  6. Sep 16, 2011 #5

    HallsofIvy

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    Re: "Solving Inequality With Complex Numbers" Question

    Right, I completely missed that.
     
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