branch cuts of atan()

Hi

the following gave me a shock:

 > atan(2)
[1] 1.107149
 > atan(2+0i)
[1] -0.4636476+0i
 >

or, perhaps more of a gotcha:

 > atan(1.0001+0i)
[1] -0.7853482+0i
 > atan(0.9999+0i)
[1] 0.7853482+0i
 >



evidently atan()'s branch cuts aren't where I thought they were.

Where do I look for documentation on this?



--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743