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One understands that for the function $\log(z^2+a^2)$ Mathematica implicitly puts the branch cuts to be starting at $\pm ia$ and going up/down respectively on the imaginary axis. The phase of this function approaches $i\pi$ on the right and above $ia$ and it approaches $-i\pi$ on the right and below $-ia$.

Now I put in the following sequence of commands,

a = 2; y = 0.0000000001; d = y ; 
z = I a + I y + d - I t;

Plot[ Im [ Log[z^2 + a^2] ], {t, 0, 5}]

I am trying to track the imaginary part of the function, Log[z^2 +a^2] as $z$ moves along a straight line parallel to and very close to the branch cuts of this function starting at $z=ia +iy + d$ (at $t=0$) to $z=-ia -iy +d$ (at $t= 2(y+a)$)

One can see that the graph goes to $\pm 2.35619 $ at $t=0$ and at $t= 2(y+a)$ - Why?

Shouldn't it be going toward $\pm \pi$ at these two values of $t$? (given that the imaginary part of the function $\log(z^2+a^2)$ approaches $\pm i\pi$ on the right and above/below $\pm ia$)


If I don't set a value for $d$ (the distance from the branchcut) and take the limit of $d \rightarrow 0$ at $t=0$ and $t=2(y+a)$ then Mathematica returns the expected values of $\pm \pi$ - but no matter how small a value of $d$ I fix (now independent of the value of $y$) the graphs still can't show the value of $\pi$ at $t=0$ though it gets the other end right.

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closed as off-topic by Jens, Michael E2, MarcoB, ilian, Öskå Aug 9 at 14:06

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It looks like $\pm\pi$ to me. V9.0.1. –  Michael E2 Apr 21 '14 at 10:12
    
@MichaelE2 Thanks. Can you kindly attach the code and the graph that you are seeing? –  user6818 Apr 21 '14 at 16:48
4  
I'm voting to close this question as off-topic because the described plot cannot be reproduced with the given code. –  Jens Aug 8 at 19:41

1 Answer 1

a = 2; y = 0.0000000001; d = y;
z = I a + I y + d - I t;

Plot[Im[Log[z^2 + a^2]], {t, -1, 5}, PlotRange -> All, 
 GridLines -> {None, {-Pi, Pi}}]

Mathematica graphics

Everything looks ok to me, considering that the imaginary part is practically negligible.

Plot[Re[z^2 + a^2], {t, -1, 5}, PlotRange -> All]
Plot[Im[z^2 + a^2], {t, -1, 5}, PlotRange -> All]

Mathematica graphics Mathematica graphics

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