theorem Th68: :: FDIFF_11:68

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