Attached Link: http://www.runtimedna.com/mod/forum/messages.php?forum_id=43&ShowMessage=186659#7

"*Anyone know how to do an inverse tangent in a shader node?*" Funny you should ask ;)

I can do arcsin and arccos, but not arctan as the approximation functions only have a valid input range of -1 to 1.

Note that it isn't perfect yet, as it doesn't have any quadrant processing.