reserve a,b,i,k,m,n for Nat;
reserve s,z for non zero Nat;
reserve c for Complex;

theorem
  for n being Integer holds n is composite implies 4 <= n
  proof
    let n be Integer;
    assume that
A1: 2 <= n and
A2: n is non prime;
    reconsider m = n as Element of NAT by A1,INT_1:3;
    assume
A3: n < 4;
    then m = 2+0 or ... or m = 2+2 by A1,NAT_1:62;
    hence thesis by A2,A3,INT_2:28,PEPIN:41;
  end;
