let X, Y be set ; :: thesis: for C being non empty set
for f1, f2 being PartFunc of C,REAL st f1 | X is constant & f2 | Y is constant holds
( (f1 + f2) | (X /\ Y) is constant & (f1 - f2) | (X /\ Y) is constant & (f1 (#) f2) | (X /\ Y) is constant )

let C be non empty set ; :: thesis: for f1, f2 being PartFunc of C,REAL st f1 | X is constant & f2 | Y is constant holds
( (f1 + f2) | (X /\ Y) is constant & (f1 - f2) | (X /\ Y) is constant & (f1 (#) f2) | (X /\ Y) is constant )

let f1, f2 be PartFunc of C,REAL; :: thesis: ( f1 | X is constant & f2 | Y is constant implies ( (f1 + f2) | (X /\ Y) is constant & (f1 - f2) | (X /\ Y) is constant & (f1 (#) f2) | (X /\ Y) is constant ) )
assume that
A1: f1 | X is constant and
A2: f2 | Y is constant ; :: thesis: ( (f1 + f2) | (X /\ Y) is constant & (f1 - f2) | (X /\ Y) is constant & (f1 (#) f2) | (X /\ Y) is constant )
consider r1 being Element of REAL such that
A3: for c being Element of C st c in X /\ (dom f1) holds
f1 . c = r1 by ;
consider r2 being Element of REAL such that
A4: for c being Element of C st c in Y /\ (dom f2) holds
f2 . c = r2 by ;
now :: thesis: for c being Element of C st c in (X /\ Y) /\ (dom (f1 + f2)) holds
(f1 + f2) . c = r1 + r2
let c be Element of C; :: thesis: ( c in (X /\ Y) /\ (dom (f1 + f2)) implies (f1 + f2) . c = r1 + r2 )
assume A5: c in (X /\ Y) /\ (dom (f1 + f2)) ; :: thesis: (f1 + f2) . c = r1 + r2
then A6: c in X /\ Y by XBOOLE_0:def 4;
then A7: c in X by XBOOLE_0:def 4;
A8: c in dom (f1 + f2) by ;
then A9: c in (dom f1) /\ (dom f2) by VALUED_1:def 1;
then c in dom f1 by XBOOLE_0:def 4;
then A10: c in X /\ (dom f1) by ;
A11: c in Y by ;
c in dom f2 by ;
then A12: c in Y /\ (dom f2) by ;
thus (f1 + f2) . c = (f1 . c) + (f2 . c) by
.= r1 + (f2 . c) by
.= r1 + r2 by ; :: thesis: verum
end;
hence (f1 + f2) | (X /\ Y) is constant by PARTFUN2:57; :: thesis: ( (f1 - f2) | (X /\ Y) is constant & (f1 (#) f2) | (X /\ Y) is constant )
now :: thesis: for c being Element of C st c in (X /\ Y) /\ (dom (f1 - f2)) holds
(f1 - f2) . c = r1 - r2
let c be Element of C; :: thesis: ( c in (X /\ Y) /\ (dom (f1 - f2)) implies (f1 - f2) . c = r1 - r2 )
assume A13: c in (X /\ Y) /\ (dom (f1 - f2)) ; :: thesis: (f1 - f2) . c = r1 - r2
then A14: c in X /\ Y by XBOOLE_0:def 4;
then A15: c in X by XBOOLE_0:def 4;
A16: c in dom (f1 - f2) by ;
then A17: c in (dom f1) /\ (dom f2) by VALUED_1:12;
then c in dom f1 by XBOOLE_0:def 4;
then A18: c in X /\ (dom f1) by ;
A19: c in Y by ;
c in dom f2 by ;
then A20: c in Y /\ (dom f2) by ;
thus (f1 - f2) . c = (f1 . c) - (f2 . c) by
.= r1 - (f2 . c) by
.= r1 - r2 by ; :: thesis: verum
end;
hence (f1 - f2) | (X /\ Y) is constant by PARTFUN2:57; :: thesis: (f1 (#) f2) | (X /\ Y) is constant
now :: thesis: for c being Element of C st c in (X /\ Y) /\ (dom (f1 (#) f2)) holds
(f1 (#) f2) . c = r1 * r2
let c be Element of C; :: thesis: ( c in (X /\ Y) /\ (dom (f1 (#) f2)) implies (f1 (#) f2) . c = r1 * r2 )
assume A21: c in (X /\ Y) /\ (dom (f1 (#) f2)) ; :: thesis: (f1 (#) f2) . c = r1 * r2
then A22: c in X /\ Y by XBOOLE_0:def 4;
c in dom (f1 (#) f2) by ;
then A23: c in (dom f1) /\ (dom f2) by VALUED_1:def 4;
then A24: c in dom f1 by XBOOLE_0:def 4;
A25: c in dom f2 by ;
c in Y by ;
then A26: c in Y /\ (dom f2) by ;
c in X by ;
then A27: c in X /\ (dom f1) by ;
thus (f1 (#) f2) . c = (f1 . c) * (f2 . c) by VALUED_1:5
.= r1 * (f2 . c) by
.= r1 * r2 by ; :: thesis: verum
end;
hence (f1 (#) f2) | (X /\ Y) is constant by PARTFUN2:57; :: thesis: verum