let m be non empty Element of NAT ; :: thesis:
let i be Element of NAT ; :: thesis:
let X be Subset of (REAL m); :: thesis:
let f, g be PartFunc of (REAL m),REAL; :: thesis:
assume AS: ( X is open & 1 <= i & i <= m & f is_partial_differentiable_on X,i & g is_partial_differentiable_on X,i ) ; :: thesis:
P1: ( X c= dom f & X c= dom g ) by AS, PDIFF734;
Q1: ( dom (f `partial| (X,i)) = X & dom (g `partial| (X,i)) = X ) by DefPDX, AS;
dom (f (#) g) = (dom f) /\ (dom g) by VALUED_1:def 4;
then P3: X c= dom (f (#) g) by P1, XBOOLE_1:19;

XX1: now :: thesis:

let x be Element of REAL m; :: thesis:
assume x in X ; :: thesis:
then ( f is_partial_differentiable_in x,i & g is_partial_differentiable_in x,i ) by AS, PDIFF734;
hence ( f (#) g is_partial_differentiable_in x,i & partdiff ((f (#) g),x,i) = ((partdiff (f,x,i)) * (g . x)) + ((f . x) * (partdiff (g,x,i))) ) by MPDIFF129; :: thesis:

end;

then P7: for x being Element of REAL m st x in X holds
f (#) g is_partial_differentiable_in x,i ;
then P8: f (#) g is_partial_differentiable_on X,i by P3, PDIFF734, AS;
then P9: dom ((f (#) g) `partial| (X,i)) = X by DefPDX;

P10: now :: thesis:

let x be Element of REAL m; :: thesis:
assume P10: x in X ; :: thesis:
then ((f (#) g) `partial| (X,i)) /. x = partdiff ((f (#) g),x,i) by P8, DefPDX;
hence ((f (#) g) `partial| (X,i)) /. x = ((partdiff (f,x,i)) * (g . x)) + ((f . x) * (partdiff (g,x,i))) by XX1, P10; :: thesis:

end;

( dom ((f `partial| (X,i)) (#) g) = (dom (f `partial| (X,i))) /\ (dom g) & dom (f (#) (g `partial| (X,i))) = (dom f) /\ (dom (g `partial| (X,i))) ) by VALUED_1:def 4;
then P12: ( dom ((f `partial| (X,i)) (#) g) = X & dom (f (#) (g `partial| (X,i))) = X ) by Q1, P1, XBOOLE_1:28;
P14: dom (((f `partial| (X,i)) (#) g) + (f (#) (g `partial| (X,i)))) = (dom ((f `partial| (X,i)) (#) g)) /\ (dom (f (#) (g `partial| (X,i)))) by VALUED_1:def 1;

now :: thesis:

let x be Element of REAL m; :: thesis:
assume A1: x in X ; :: thesis:
thus ((f (#) g) `partial| (X,i)) . x = ((f (#) g) `partial| (X,i)) /. x by A1, P9, PARTFUN1:def 6
.= ((partdiff (f,x,i)) * (g . x)) + ((f . x) * (partdiff (g,x,i))) by P10, A1
.= (((f `partial| (X,i)) /. x) * (g . x)) + ((f . x) * (partdiff (g,x,i))) by A1, DefPDX, AS
.= (((f `partial| (X,i)) /. x) * (g . x)) + ((f . x) * ((g `partial| (X,i)) /. x)) by A1, DefPDX, AS
.= (((f `partial| (X,i)) . x) * (g . x)) + ((f . x) * ((g `partial| (X,i)) /. x)) by A1, Q1, PARTFUN1:def 6
.= (((f `partial| (X,i)) . x) * (g . x)) + ((f . x) * ((g `partial| (X,i)) . x)) by A1, Q1, PARTFUN1:def 6
.= (((f `partial| (X,i)) (#) g) . x) + ((f . x) * ((g `partial| (X,i)) . x)) by VALUED_1:5
.= (((f `partial| (X,i)) (#) g) . x) + ((f (#) (g `partial| (X,i))) . x) by VALUED_1:5
.= (((f `partial| (X,i)) (#) g) + (f (#) (g `partial| (X,i)))) . x by A1, P14, P12, VALUED_1:def 1 ; :: thesis:

end;

hence ( f (#) g is_partial_differentiable_on X,i & (f (#) g) `partial| (X,i) = ((f `partial| (X,i)) (#) g) + (f (#) (g `partial| (X,i))) & ( for x being Element of REAL m st x in X holds
((f (#) g) `partial| (X,i)) /. x = ((partdiff (f,x,i)) * (g . x)) + ((f . x) * (partdiff (g,x,i))) ) ) by P7, P10, P3, PDIFF734, AS, P9, P12, P14, PARTFUN1:5; :: thesis: