let Z be open Subset of REAL; :: thesis: ( Z c= dom (arctan * arccot) & Z c= ].(- 1),1.[ & ( for x being Real st x in Z holds
( arccot . x > - 1 & arccot . x < 1 ) ) implies ( arctan * arccot is_differentiable_on Z & ( for x being Real st x in Z holds
((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) ) ) )
assume that
A1: Z c= dom (arctan * arccot) and
A2: Z c= ].(- 1),1.[ and
A3: for x being Real st x in Z holds
( arccot . x > - 1 & arccot . x < 1 ) ; :: thesis: ( arctan * arccot is_differentiable_on Z & ( for x being Real st x in Z holds
((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) ) )
A4: for x being Real st x in Z holds
arctan * arccot is_differentiable_in x

proof

let x be Real; :: thesis: ( x in Z implies arctan * arccot is_differentiable_in x )
assume A5: x in Z ; :: thesis: arctan * arccot is_differentiable_in x
then A6: ( arccot . x > - 1 & arccot . x < 1 ) by A3;
arccot is_differentiable_on Z by A2, SIN_COS9:82;
then arccot is_differentiable_in x by A5, FDIFF_1:9;
hence arctan * arccot is_differentiable_in x by A6, SIN_COS9:85; :: thesis: verum

end;

then A7: arctan * arccot is_differentiable_on Z by A1, FDIFF_1:9;
for x being Real st x in Z holds
((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2))))

proof

let x be Real; :: thesis: ( x in Z implies ((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) )
assume A8: x in Z ; :: thesis: ((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2))))
then A9: ( arccot . x > - 1 & arccot . x < 1 ) by A3;
A10: arccot is_differentiable_on Z by A2, SIN_COS9:82;
then A11: arccot is_differentiable_in x by A8, FDIFF_1:9;
((arctan * arccot) `| Z) . x = diff ((arctan * arccot),x) by A7, A8, FDIFF_1:def 7
.= (diff (arccot,x)) / (1 + ((arccot . x) ^2)) by A11, A9, SIN_COS9:85
.= ((arccot `| Z) . x) / (1 + ((arccot . x) ^2)) by A8, A10, FDIFF_1:def 7
.= (- (1 / (1 + (x ^2)))) / (1 + ((arccot . x) ^2)) by A2, A8, SIN_COS9:82
.= - ((1 / (1 + (x ^2))) / (1 + ((arccot . x) ^2)))
.= - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) by XCMPLX_1:78 ;
hence ((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) ; :: thesis: verum

end;

hence ( arctan * arccot is_differentiable_on Z & ( for x being Real st x in Z holds
((arctan * arccot) `| Z) . x = - (1 / ((1 + (x ^2)) * (1 + ((arccot . x) ^2)))) ) ) by A1, A4, FDIFF_1:9; :: thesis: verum