let n be Element of NAT ; :: thesis: for h, x being Real
for f1, f2 being Function of REAL ,REAL holds ((fdif (f1 - f2),h) . (n + 1)) . x = (((fdif f1,h) . (n + 1)) . x) - (((fdif f2,h) . (n + 1)) . x)
let h, x be Real; :: thesis: for f1, f2 being Function of REAL ,REAL holds ((fdif (f1 - f2),h) . (n + 1)) . x = (((fdif f1,h) . (n + 1)) . x) - (((fdif f2,h) . (n + 1)) . x)
let f1, f2 be Function of REAL ,REAL ; :: thesis: ((fdif (f1 - f2),h) . (n + 1)) . x = (((fdif f1,h) . (n + 1)) . x) - (((fdif f2,h) . (n + 1)) . x)
defpred S₁[ Element of NAT ] means for x being Real holds ((fdif (f1 - f2),h) . ($1 + 1)) . x = (((fdif f1,h) . ($1 + 1)) . x) - (((fdif f2,h) . ($1 + 1)) . x);
A1: S₁[ 0 ]

proof

let x be Real; :: thesis: ((fdif (f1 - f2),h) . (0 + 1)) . x = (((fdif f1,h) . (0 + 1)) . x) - (((fdif f2,h) . (0 + 1)) . x)
x in REAL ;
then ( x in dom f1 & x in dom f2 ) by FUNCT_2:def 1;
then x in (dom f1) /\ (dom f2) by XBOOLE_0:def 4;
then A2: x in dom (f1 - f2) by VALUED_1:12;
x + h in REAL ;
then ( x + h in dom f1 & x + h in dom f2 ) by FUNCT_2:def 1;
then x + h in (dom f1) /\ (dom f2) by XBOOLE_0:def 4;
then A3: x + h in dom (f1 - f2) by VALUED_1:12;
((fdif (f1 - f2),h) . (0 + 1)) . x = (fD ((fdif (f1 - f2),h) . 0 ),h) . x by Def6
.= (fD (f1 - f2),h) . x by Def6
.= ((f1 - f2) . (x + h)) - ((f1 - f2) . x) by Th3
.= ((f1 . (x + h)) - (f2 . (x + h))) - ((f1 - f2) . x) by A3, VALUED_1:13
.= ((f1 . (x + h)) - (f2 . (x + h))) - ((f1 . x) - (f2 . x)) by A2, VALUED_1:13
.= ((f1 . (x + h)) - (f1 . x)) - ((f2 . (x + h)) - (f2 . x))
.= ((fD f1,h) . x) - ((f2 . (x + h)) - (f2 . x)) by Th3
.= ((fD f1,h) . x) - ((fD f2,h) . x) by Th3
.= ((fD ((fdif f1,h) . 0 ),h) . x) - ((fD f2,h) . x) by Def6
.= ((fD ((fdif f1,h) . 0 ),h) . x) - ((fD ((fdif f2,h) . 0 ),h) . x) by Def6
.= (((fdif f1,h) . (0 + 1)) . x) - ((fD ((fdif f2,h) . 0 ),h) . x) by Def6
.= (((fdif f1,h) . (0 + 1)) . x) - (((fdif f2,h) . (0 + 1)) . x) by Def6 ;
hence ((fdif (f1 - f2),h) . (0 + 1)) . x = (((fdif f1,h) . (0 + 1)) . x) - (((fdif f2,h) . (0 + 1)) . x) ; :: thesis: verum

end;

A4: for k being Element of NAT st S₁[k] holds
S₁[k + 1]

proof

let k be Element of NAT ; :: thesis: ( S₁[k] implies S₁[k + 1] )
assume A5: for x being Real holds ((fdif (f1 - f2),h) . (k + 1)) . x = (((fdif f1,h) . (k + 1)) . x) - (((fdif f2,h) . (k + 1)) . x) ; :: thesis: S₁[k + 1]
let x be Real; :: thesis: ((fdif (f1 - f2),h) . ((k + 1) + 1)) . x = (((fdif f1,h) . ((k + 1) + 1)) . x) - (((fdif f2,h) . ((k + 1) + 1)) . x)
A6: ( ((fdif (f1 - f2),h) . (k + 1)) . x = (((fdif f1,h) . (k + 1)) . x) - (((fdif f2,h) . (k + 1)) . x) & ((fdif (f1 - f2),h) . (k + 1)) . (x + h) = (((fdif f1,h) . (k + 1)) . (x + h)) - (((fdif f2,h) . (k + 1)) . (x + h)) ) by A5;
A7: (fdif (f1 - f2),h) . (k + 1) is Function of REAL ,REAL by Th2;
A8: (fdif f2,h) . (k + 1) is Function of REAL ,REAL by Th2;
A9: (fdif f1,h) . (k + 1) is Function of REAL ,REAL by Th2;
((fdif (f1 - f2),h) . ((k + 1) + 1)) . x = (fD ((fdif (f1 - f2),h) . (k + 1)),h) . x by Def6
.= (((fdif (f1 - f2),h) . (k + 1)) . (x + h)) - (((fdif (f1 - f2),h) . (k + 1)) . x) by A7, Th3
.= ((((fdif f1,h) . (k + 1)) . (x + h)) - (((fdif f1,h) . (k + 1)) . x)) - ((((fdif f2,h) . (k + 1)) . (x + h)) - (((fdif f2,h) . (k + 1)) . x)) by A6
.= ((fD ((fdif f1,h) . (k + 1)),h) . x) - ((((fdif f2,h) . (k + 1)) . (x + h)) - (((fdif f2,h) . (k + 1)) . x)) by A9, Th3
.= ((fD ((fdif f1,h) . (k + 1)),h) . x) - ((fD ((fdif f2,h) . (k + 1)),h) . x) by A8, Th3
.= (((fdif f1,h) . ((k + 1) + 1)) . x) - ((fD ((fdif f2,h) . (k + 1)),h) . x) by Def6
.= (((fdif f1,h) . ((k + 1) + 1)) . x) - (((fdif f2,h) . ((k + 1) + 1)) . x) by Def6 ;
hence ((fdif (f1 - f2),h) . ((k + 1) + 1)) . x = (((fdif f1,h) . ((k + 1) + 1)) . x) - (((fdif f2,h) . ((k + 1) + 1)) . x) ; :: thesis: verum

end;

for n being Element of NAT holds S₁[n] from NAT_1:sch 1(A1, A4);
hence ((fdif (f1 - f2),h) . (n + 1)) . x = (((fdif f1,h) . (n + 1)) . x) - (((fdif f2,h) . (n + 1)) . x) ; :: thesis: verum