atan(a*x + b)
a -------------- 2 1 + (a*x + b)
2 -2*a *(b + a*x) ----------------- 2 / 2\ \1 + (b + a*x) /
/ 2 \ 3 | 4*(b + a*x) | 2*a *|-1 + --------------| | 2| \ 1 + (b + a*x) / -------------------------- 2 / 2\ \1 + (b + a*x) /