assume
ex k being Nat st ( k >= n & x in A . k )
; :: thesis: ex k being Nat st x in(A ^\ n). k then consider k being Nat such that A7:
( k >= n & x in A . k )
; consider knat being Nat such that A8:
k = n + knat
byA7, NAT_1:10; reconsider knat = knat as Nat ; A9:
( x in A . k implies x in(A ^\ n). knat )
byA8, NAT_1:def 3; thus
ex k being Nat st x in(A ^\ n). k
byA7, A9; :: thesis: verum
end;
A10:
for x being object holds ( ( for m being Nat ex n being Nat st ( n >= m & x in A . n ) ) iff for m being Nat ex n being Nat st x in A .(m + n) )
let x be object ; :: thesis: ( ex n being Nat st for k being Nat st k >= n holds x in A . k iff ex n being Nat st for k being Nat holds x in A .(n + k) )
hereby :: thesis: ( ex n being Nat st for k being Nat holds x in A .(n + k) implies ex n being Nat st for k being Nat st k >= n holds x in A . k )
assume
ex n being Nat st for k being Nat st k >= n holds x in A . k
; :: thesis: ex n being Nat st for k being Nat holds x in A .(n + k) then consider n being Nat such that A20:
for k being Nat st k >= n holds x in A . k
;
for k being Nat holds x in A .(n + k)byA20, NAT_1:11; hence
ex n being Nat st for k being Nat holds x in A .(n + k)
; :: thesis: verum
end;
assume
ex n being Nat st for k being Nat holds x in A .(n + k)
; :: thesis: ex n being Nat st for k being Nat st k >= n holds x in A . k then consider n being Nat such that A21:
for k being Nat holds x in A .(n + k)
;
for k being Nat st k >= n holds x in A . k