let r be Real; :: thesis: for Z being open Subset of REAL
for f being PartFunc of REAL ,REAL st Z c= dom (r (#) f) & f is_differentiable_on Z holds
( r (#) f is_differentiable_on Z & ( for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x) ) )
let Z be open Subset of REAL ; :: thesis: for f being PartFunc of REAL ,REAL st Z c= dom (r (#) f) & f is_differentiable_on Z holds
( r (#) f is_differentiable_on Z & ( for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x) ) )
let f be PartFunc of REAL ,REAL ; :: thesis: ( Z c= dom (r (#) f) & f is_differentiable_on Z implies ( r (#) f is_differentiable_on Z & ( for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x) ) ) )
assume that
A1: Z c= dom (r (#) f) and
A2: f is_differentiable_on Z ; :: thesis: ( r (#) f is_differentiable_on Z & ( for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x) ) )

now

let x0 be Real; :: thesis: ( x0 in Z implies r (#) f is_differentiable_in x0 )
assume x0 in Z ; :: thesis: r (#) f is_differentiable_in x0
then f is_differentiable_in x0 by A2, Th16;
hence r (#) f is_differentiable_in x0 by Th23; :: thesis: verum

end;

hence A3: r (#) f is_differentiable_on Z by A1, Th16; :: thesis: for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x)

now

let x be Real; :: thesis: ( x in Z implies ((r (#) f) `| Z) . x = r * (diff f,x) )
assume A4: x in Z ; :: thesis: ((r (#) f) `| Z) . x = r * (diff f,x)
then A5: f is_differentiable_in x by A2, Th16;
thus ((r (#) f) `| Z) . x = diff (r (#) f),x by A3, A4, Def8
.= r * (diff f,x) by A5, Th23 ; :: thesis: verum

end;

hence for x being Real st x in Z holds
((r (#) f) `| Z) . x = r * (diff f,x) ; :: thesis: verum