theorem :: SIN_COS9:119

for Z being open Subset of REAL st Z c= dom (exp_R * arctan) & Z c= ].(- 1),1.[ holds
( exp_R * arctan is_differentiable_on Z & ( for x being Real st x in Z holds
((exp_R * arctan) `| Z) . x = (exp_R . (arctan . x)) / (1 + (x ^2)) ) )