let A be symmetrical Subset of COMPLEX; :: thesis: ( A c= dom cot implies cot is_odd_on A )
assume A1: A c= dom cot ; :: thesis: cot is_odd_on A
A2: dom (cot | A) = A by A1, RELAT_1:62;
A3: for x being Real st x in A holds
cot . (- x) = - (cot . x)

proof

let x be Real; :: thesis: ( x in A implies cot . (- x) = - (cot . x) )
assume A4: x in A ; :: thesis: cot . (- x) = - (cot . x)
then A5: sin . x <> 0 by A1, FDIFF_8:2;
- x in A by A4, Def1;
then cot . (- x) = cot (- x) by A1, FDIFF_8:2, SIN_COS9:16
.= - (cot x) by SIN_COS4:3
.= - (cot . x) by A5, SIN_COS9:16 ;
hence cot . (- x) = - (cot . x) ; :: thesis: verum

end;

for x being Real st x in dom (cot | A) & - x in dom (cot | A) holds
(cot | A) . (- x) = - ((cot | A) . x)

proof

let x be Real; :: thesis: ( x in dom (cot | A) & - x in dom (cot | A) implies (cot | A) . (- x) = - ((cot | A) . x) )
assume that
A6: x in dom (cot | A) and
A7: - x in dom (cot | A) ; :: thesis: (cot | A) . (- x) = - ((cot | A) . x)
reconsider x = x as Element of REAL by XREAL_0:def 1;
(cot | A) . (- x) = (cot | A) /. (- x) by A7, PARTFUN1:def 6
.= cot /. (- x) by A1, A2, A7, PARTFUN2:17
.= cot . (- x) by A1, A7, PARTFUN1:def 6
.= - (cot . x) by A3, A6
.= - (cot /. x) by A1, A6, PARTFUN1:def 6
.= - ((cot | A) /. x) by A1, A2, A6, PARTFUN2:17
.= - ((cot | A) . x) by A6, PARTFUN1:def 6 ;
hence (cot | A) . (- x) = - ((cot | A) . x) ; :: thesis: verum

end;

then ( cot | A is with_symmetrical_domain & cot | A is quasi_odd ) by A2;
hence cot is_odd_on A by A1; :: thesis: verum