theorem Th4:
  i <> -1 & i <> 1 & i divides j implies not i divides j-1
  proof
    assume
A1: i <> -1 & i <> 1;
    assume i divides j & i divides j-1;
    then i divides j-1-j by INT_5:1;
    hence contradiction by A1,INT_2:13;
  end;
