Complex analysis - argument principle

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Discussion Overview

The discussion revolves around the argument principle in complex analysis, specifically addressing the calculation of the change in the argument of a function as it traverses a closed contour. Participants explore the implications of obtaining a non-integer multiple of \(2\pi\) for the change in argument and how this relates to the number of zeros and holes of the function within the contour.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions what to do when the change in argument \(h(z)\) is \(9\pi/2\), suggesting rounding to determine the number of zeros.
  • Another participant asks for examples where the change in argument is not an integer multiple of \(2\pi\).
  • A participant expresses confusion about how the change in argument could be anything other than an integral multiple of \(2\pi\) when traversing a closed loop.
  • One participant describes their process for determining the number of zeros of the function \(f(z) = z^9 + 5z^2 + 3\) in the first quadrant, initially arriving at \(9\pi/2\) and questioning if they should instead get \(8\pi/2\).
  • Another participant suggests that the confusion may stem from not accounting for the argument's behavior along specific segments of the contour, particularly from \(iR\) to \(0\).
  • A later reply indicates that the participant is now confident in their understanding, having adjusted their calculation to account for the argument's path and arriving at a total change of \(4\pi\).
  • One participant comments on the frequency of questions being asked, suggesting that the volume of inquiries may indicate a lack of thorough consideration before posting.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the change in argument and its implications. There is no consensus on how to interpret non-integer multiples of \(2\pi\) in this context, and the discussion remains unresolved regarding the initial confusion about the calculations.

Contextual Notes

Some participants note potential misunderstandings in the contour integration process and the behavior of the function along different segments, but these issues are not fully resolved.

sweetvirgogirl
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(changes in arg h (z) as z traverses lambda)/(2pi) =
# of zeroes of h inside lambda +
# of holes of h inside lambda

now the doubt i have is what happens when the change i get in h (z) is say 9 pi/2 ... because then i would have a 2.5 on left side of the eqn ... so do i round it up and and say the function will have 2 zeros? (assuming it has no holes)
 
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Can you come up with an example where the change in arg h(z) around a closed curve is 9pi/2? Or anything that isn't an integer multiple of 2pi?
 
how can the change in argument, i.e. angle, be other than an integral multiple of 2pi, when you go around a closed loop?
 
okay ... so i believe i am doing something wrong then ...
f(z) = z^9 +5z^2 + 3
(i have to determine number of zeroes in the first quadrant)

i came up with the 9pi/2 ... i am sure i am doing something wrong ... i should be getting 8 pi/2, right?
 
It would help if you show your work, but I have a guess where the problem is. You are probably looking at the contour from 0 to R, then along the circle of radius R centered at the origin to i*R, then back to zero, (for some R sufficiently large). I don't think you took into account what happens to the argument from i*R to 0. What is f doing along this segment?
 
shmoe said:
It would help if you show your work, but I have a guess where the problem is. You are probably looking at the contour from 0 to R, then along the circle of radius R centered at the origin to i*R, then back to zero, (for some R sufficiently large). I don't think you took into account what happens to the argument from i*R to 0. What is f doing along this segment?
uhh ... sorry ...
now i am getting the answer quite fine ... just want to make sure what i am doing is right ...
so it travels 9pi/2 counting 0 to R and along the radius of R centered at origin... to iR ... but when it reaches iR, it travel clockwise to land on the positive real axis ... so i subtract that from 9 pi/2 ... which means in total... it has traveled 4 pi
 
That's essentially it. I'd expect more detail on the changes over the pieces of the contour, I assume you have but haven't posted (which is fine if you're happy with that part!).
 
sweetness and light, are you actually thinking about these many questions before asking them? it just seems that is unlikely given how many questions you are asking per day on different topics.
just a suggestion.
 

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