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Why ArcTan[x,y]-ArcTan[x,-y] = 2 ArcTan[x,y] is not equivalent in the WL?

Posted 8 years ago

I'm working with integrals that yield pairs of terms of the form ArcTan[x,y]-ArcTan[x,-y] or equivalent expressions involving Arg or complex Log. For Mathematica's choice of the branch cut, this is equal to 2 ArcTan[x,y]. However, it appears that the only way to achieve this simplification, or even to get Mathematica to acknowledge that these are equivalent, is to use a custom transformation function like

negt[ArcTan[x_, y_]] := -ArcTan[x, -y]

in Simplify or FullSimplify.

Have I missed something?

POSTED BY: John Doty
2 Replies
In[1]:= $Assumptions = {x > 0};     Simplify[ArcTan[x, y] - ArcTan[x, -y] - 2 ArcTan[x, y]]
Out[1]= 0
POSTED BY: Rolf Mertig
In[2]:= ArcTan[x, y] - ArcTan[x, -y] - 2 ArcTan[x, y] /. {x -> -1, y -> 0}

Out[2]= -2 Pi
POSTED BY: Ilian Gachevski
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