Complex Analysis - sqrt(z^2 + 1) function behavior

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Homework Statement


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Homework Equations


The relevant equation is that sqrt(z) = e^(1/2 log z) and the principal branch is from (-pi, pi]

The Attempt at a Solution


The solution is provided, since this isn't a homework problem (I was told to post it here anyway). I don't understand why the number z^2 + 1 crosses the negative real half-line in the counterclockwise direction as z crosses (i, i inf). Is it because z^2 + 1 where z = i is 0? I would think the argument of z^2 + 1 should be around pi in order to cross the negative real half line, but I'm not sure. Also, I'm not sure how they said the value jumps from it to -it when the half lines are crossed.
 

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Answers and Replies

  • #2
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Let ##z=2i+\epsilon## for example, then ##z^2+1 = -3+\epsilon^2 + 2 i \epsilon##, just a little bit above the discontinuity in the square root. The argument is close to pi but smaller, the square root will be close to the positive imaginary axis.
For ##z'=2i-\epsilon## we get ##z'^2+1 = -3+\epsilon^2 - 2 i \epsilon##, just a little bit below the discontinuity in the square root. The argument is close to pi but larger, the square root will be close to the negative imaginary axis.
 

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