let f be Function; :: thesis: for n being Nat st rng f c= dom f holds
( dom (iter f,n) = dom f & rng (iter f,n) c= dom f )
let n be Nat; :: thesis: ( rng f c= dom f implies ( dom (iter f,n) = dom f & rng (iter f,n) c= dom f ) )
defpred S₁[ Nat] means ( dom (iter f,$1) = dom f & rng (iter f,$1) c= dom f );
assume rng f c= dom f ; :: thesis: ( dom (iter f,n) = dom f & rng (iter f,n) c= dom f )
then iter f,0 = id (dom f) by Lm4;
then A1: S₁[ 0 ] by RELAT_1:71;
A2: for k being Nat st S₁[k] holds
S₁[k + 1]

proof

let k be Nat; :: thesis: ( S₁[k] implies S₁[k + 1] )
assume A3: ( dom (iter f,k) = dom f & rng (iter f,k) c= dom f ) ; :: thesis: S₁[k + 1]
( iter f,(k + 1) = f * (iter f,k) & iter f,(k + 1) = (iter f,k) * f ) by Th71, Th73;
then ( dom (iter f,(k + 1)) = dom (iter f,k) & rng (iter f,(k + 1)) c= rng (iter f,k) ) by A3, RELAT_1:45, RELAT_1:46;
hence S₁[k + 1] by A3, XBOOLE_1:1; :: thesis: verum

end;

for k being Nat holds S₁[k] from NAT_1:sch 2(A1, A2);
hence ( dom (iter f,n) = dom f & rng (iter f,n) c= dom f ) ; :: thesis: verum