atan(4*x)
d --(atan(4*x)) dx
-128*x
------------
2
/ 2\
\1 + 16*x /
/ 2 \
| 64*x |
128*|-1 + ---------|
| 2|
\ 1 + 16*x /
--------------------
2
/ 2\
\1 + 16*x /