theorem Th23: :: INTEGR13:23

for Z being open Subset of REAL st Z c= dom (exp_R * cot) holds
( - (exp_R * cot) is_differentiable_on Z & ( for x being Real st x in Z holds
((- (exp_R * cot)) `| Z) . x = (exp_R . (cot . x)) / ((sin . x) ^2) ) )